diff --git a/modules/nf-core/gcta/addgrms/environment.yml b/modules/nf-core/gcta/addgrms/environment.yml
new file mode 100644
index 000000000000..3e22ea7b9f20
--- /dev/null
+++ b/modules/nf-core/gcta/addgrms/environment.yml
@@ -0,0 +1,7 @@
+---
+# yaml-language-server: $schema=https://raw.githubusercontent.com/nf-core/modules/master/modules/environment-schema.json
+channels:
+  - conda-forge
+  - bioconda
+dependencies:
+  - bioconda::gcta=1.94.1
diff --git a/modules/nf-core/gcta/addgrms/main.nf b/modules/nf-core/gcta/addgrms/main.nf
new file mode 100644
index 000000000000..9148ce4ef9f2
--- /dev/null
+++ b/modules/nf-core/gcta/addgrms/main.nf
@@ -0,0 +1,38 @@
+process GCTA_ADDGRMS {
+    tag "${meta.id}"
+    label 'process_medium'
+    conda "${moduleDir}/environment.yml"
+    container "${workflow.containerEngine in ['singularity', 'apptainer'] && !task.ext.singularity_pull_docker_container
+        ? 'https://community-cr-prod.seqera.io/docker/registry/v2/blobs/sha256/46/46b0d05f0daa47561d87d2a9cac5e51edc2c78e26f1bbab439c688386241a274/data'
+        : 'community.wave.seqera.io/library/gcta:1.94.1--9bc35dc424fcf6e9'}"
+
+    input:
+    tuple val(meta), path(mgrm_file), path(grm_files)
+
+    output:
+    tuple val(meta), path("*.grm.*"), emit: combined_grm
+    tuple val("${task.process}"), val("gcta"), eval("gcta --version | sed -En 's/^[*] version v([0-9.]*).*/\\1/p'"), emit: versions_gcta, topic: versions
+
+    when:
+    task.ext.when == null || task.ext.when
+
+    script:
+    def args = task.ext.args ?: ''
+    def prefix = task.ext.prefix ?: "${meta.id}"
+    """
+    gcta \\
+        --mgrm ${mgrm_file} \\
+        --make-grm \\
+        --out ${prefix} \\
+        --thread-num ${task.cpus} \\
+        ${args}
+    """
+
+    stub:
+    def prefix = task.ext.prefix ?: "${meta.id}"
+    """
+    touch ${prefix}.grm.id
+    touch ${prefix}.grm.bin
+    touch ${prefix}.grm.N.bin
+    """
+}
diff --git a/modules/nf-core/gcta/addgrms/meta.yml b/modules/nf-core/gcta/addgrms/meta.yml
new file mode 100644
index 000000000000..a39563ee1ec4
--- /dev/null
+++ b/modules/nf-core/gcta/addgrms/meta.yml
@@ -0,0 +1,75 @@
+# yaml-language-server: $schema=https://raw.githubusercontent.com/nf-core/modules/master/modules/meta-schema.json
+name: "gcta_addgrms"
+description: Combine multiple GRMs listed in an MGRM manifest into a single dense GRM
+keywords:
+  - gcta
+  - genome-wide complex trait analysis
+  - grm
+  - genetic relationship matrix
+  - genetics
+tools:
+  - "gcta":
+      description: "Genome-wide Complex Trait Analysis (GCTA) estimates genetic relationships, variance components, and association statistics from genome-wide data."
+      homepage: "https://yanglab.westlake.edu.cn/software/gcta/"
+      documentation: "https://yanglab.westlake.edu.cn/software/gcta/static/gcta_doc_latest.pdf"
+      tool_dev_url: "https://yanglab.westlake.edu.cn/software/gcta/"
+      licence: ["GPL-3.0-only"]
+      identifier: "biotools:gcta"
+
+input:
+  - - meta:
+        type: map
+        description: |
+          Groovy map containing combined GRM metadata
+          e.g. `[ id:'plink_simulated' ]`
+    - mgrm_file:
+        type: file
+        description: MGRM manifest listing the GRM prefixes to combine
+        pattern: "*.mgrm"
+        ontologies:
+          - edam: "http://edamontology.org/format_2330"
+    - grm_files:
+        type: file
+        description: GRM sidecar files referenced by `mgrm_file`
+        pattern: "*"
+        ontologies: []
+
+output:
+  combined_grm:
+    - - meta:
+          type: map
+          description: |
+            Groovy map containing combined GRM metadata
+            e.g. `[ id:'plink_simulated' ]`
+      - "*.grm.*":
+          type: file
+          description: Combined dense GRM sidecar files
+          pattern: "*.grm.{id,bin,N.bin}"
+          ontologies: []
+  versions_gcta:
+    - - "${task.process}":
+          type: string
+          description: The process the version was collected from
+      - "gcta":
+          type: string
+          description: The tool name
+      - "gcta --version | sed -En 's/^[*] version v([0-9.]*).*/\\1/p'":
+          type: eval
+          description: The command used to retrieve the GCTA version
+
+topics:
+  versions:
+    - - ${task.process}:
+          type: string
+          description: The process the version was collected from
+      - gcta:
+          type: string
+          description: The tool name
+      - "gcta --version | sed -En 's/^[*] version v([0-9.]*).*/\\1/p'":
+          type: eval
+          description: The command used to retrieve the GCTA version
+
+authors:
+  - "@lyh970817"
+maintainers:
+  - "@lyh970817"
diff --git a/modules/nf-core/gcta/addgrms/tests/main.nf.test b/modules/nf-core/gcta/addgrms/tests/main.nf.test
new file mode 100644
index 000000000000..cb5927caf816
--- /dev/null
+++ b/modules/nf-core/gcta/addgrms/tests/main.nf.test
@@ -0,0 +1,162 @@
+nextflow_process {
+
+    name "Test Process GCTA_ADDGRMS"
+    script "../main.nf"
+    process "GCTA_ADDGRMS"
+
+    tag "modules"
+    tag "modules_nfcore"
+    tag "gcta"
+    tag "gcta/addgrms"
+    tag "gcta/makegrmpart"
+
+    setup {
+        run("GCTA_MAKEGRMPART", alias: "GCTA_MAKEGRMPART_LDMS1") {
+            script "../../makegrmpart/main.nf"
+            process {
+                """
+                file('plink_simulated_ldms1.mbfile').text = 'plink_simulated\\n'
+
+                def ldms1BimFile = file(params.modules_testdata_base_path + 'genomics/homo_sapiens/popgen/plink_simulated.bim', checkIfExists: true)
+                def ldms1ExtractSnps = ldms1BimFile.readLines()
+                    .take(10)
+                    .collect { row -> row.trim().split(/\\s+/)[1] }
+                    .join('\\n') + '\\n'
+                file('plink_simulated_ldms1.snps.txt').text = ldms1ExtractSnps
+
+                input[0] = [
+                    [ id:'plink_simulated_ldms1' ],
+                    1,
+                    1,
+                    file('plink_simulated_ldms1.mbfile'),
+                    [
+                        file(params.modules_testdata_base_path + 'genomics/homo_sapiens/popgen/plink_simulated.bed', checkIfExists: true)
+                    ],
+                    [
+                        ldms1BimFile
+                    ],
+                    [
+                        file(params.modules_testdata_base_path + 'genomics/homo_sapiens/popgen/plink_simulated.fam', checkIfExists: true)
+                    ]
+                ]
+                input[1] = [[ id:'plink_simulated_ldms1' ], file('plink_simulated_ldms1.snps.txt')]
+                """
+            }
+        }
+
+        run("GCTA_MAKEGRMPART", alias: "GCTA_MAKEGRMPART_LDMS2") {
+            script "../../makegrmpart/main.nf"
+            process {
+                """
+                file('plink_simulated_ldms2.mbfile').text = 'plink_simulated\\n'
+
+                def ldms2BimFile = file(params.modules_testdata_base_path + 'genomics/homo_sapiens/popgen/plink_simulated.bim', checkIfExists: true)
+                def ldms2ExtractSnps = ldms2BimFile.readLines()
+                    .drop(10)
+                    .take(10)
+                    .collect { row -> row.trim().split(/\\s+/)[1] }
+                    .join('\\n') + '\\n'
+                file('plink_simulated_ldms2.snps.txt').text = ldms2ExtractSnps
+
+                input[0] = [
+                    [ id:'plink_simulated_ldms2' ],
+                    1,
+                    1,
+                    file('plink_simulated_ldms2.mbfile'),
+                    [
+                        file(params.modules_testdata_base_path + 'genomics/homo_sapiens/popgen/plink_simulated.bed', checkIfExists: true)
+                    ],
+                    [
+                        ldms2BimFile
+                    ],
+                    [
+                        file(params.modules_testdata_base_path + 'genomics/homo_sapiens/popgen/plink_simulated.fam', checkIfExists: true)
+                    ]
+                ]
+                input[1] = [[ id:'plink_simulated_ldms2' ], file('plink_simulated_ldms2.snps.txt')]
+                """
+            }
+        }
+    }
+
+    test("homo_sapiens popgen - merge dense GRMs from mgrm") {
+        when {
+            process {
+                """
+                mgrm_file = Channel
+                    .of('plink_simulated_ldms1.part_1_1\\nplink_simulated_ldms2.part_1_1')
+                    .collectFile(name:'plink_simulated_ldms.mgrm', newLine: true)
+
+                grm_files = GCTA_MAKEGRMPART_LDMS1.out.grm_files
+                    .mix(GCTA_MAKEGRMPART_LDMS2.out.grm_files)
+                    .map { meta, grm_files, nparts_gcta, part_gcta_job -> grm_files }
+                    .collect()
+                    .map { bundles -> bundles.flatten().sort { it.name } }
+
+                input[0] = mgrm_file
+                    .combine(grm_files)
+                    .map { row -> [[ id:'plink_simulated_ldms' ], row[0], row[1..-1]] }
+                """
+            }
+        }
+
+        then {
+            assertAll(
+                { assert process.success },
+                { assert process.out.combined_grm.size() == 1 },
+                { assert process.out.combined_grm.get(0).get(0).id == "plink_simulated_ldms" },
+                {
+                    assert process.out.combined_grm.get(0).get(1).collect { file(it).name }.toSet() == [
+                        'plink_simulated_ldms.grm.id',
+                        'plink_simulated_ldms.grm.bin',
+                        'plink_simulated_ldms.grm.N.bin'
+                    ] as Set
+                },
+                {
+                    assert snapshot(
+                        process.out.combined_grm,
+                        process.out.findAll { key, val -> key.startsWith('versions') }
+                    ).match()
+                }
+            )
+        }
+    }
+
+    test("homo_sapiens popgen - merge dense GRMs from mgrm - stub") {
+        options "-stub"
+
+        when {
+            process {
+                """
+                mgrm_file = Channel
+                    .of('plink_simulated_ldms1.part_1_1\\nplink_simulated_ldms2.part_1_1')
+                    .collectFile(name:'plink_simulated_ldms.mgrm', newLine: true)
+
+                grm_files = GCTA_MAKEGRMPART_LDMS1.out.grm_files
+                    .mix(GCTA_MAKEGRMPART_LDMS2.out.grm_files)
+                    .map { meta, grm_files, nparts_gcta, part_gcta_job -> grm_files }
+                    .collect()
+                    .map { bundles -> bundles.flatten().sort { it.name } }
+
+                input[0] = mgrm_file
+                    .combine(grm_files)
+                    .map { row -> [[ id:'plink_simulated_ldms' ], row[0], row[1..-1]] }
+                """
+            }
+        }
+
+        then {
+            assertAll(
+                { assert process.success },
+                { assert process.out.combined_grm.size() == 1 },
+                { assert process.out.combined_grm.get(0).get(0).id == "plink_simulated_ldms" },
+                {
+                    assert snapshot(
+                        process.out.combined_grm,
+                        process.out.findAll { key, val -> key.startsWith('versions') }
+                    ).match()
+                }
+            )
+        }
+    }
+}
diff --git a/modules/nf-core/gcta/addgrms/tests/main.nf.test.snap b/modules/nf-core/gcta/addgrms/tests/main.nf.test.snap
new file mode 100644
index 000000000000..d93d9255ef2f
--- /dev/null
+++ b/modules/nf-core/gcta/addgrms/tests/main.nf.test.snap
@@ -0,0 +1,62 @@
+{
+    "homo_sapiens popgen - merge dense GRMs from mgrm - stub": {
+        "content": [
+            [
+                [
+                    {
+                        "id": "plink_simulated_ldms"
+                    },
+                    [
+                        "plink_simulated_ldms.grm.N.bin:md5,d41d8cd98f00b204e9800998ecf8427e",
+                        "plink_simulated_ldms.grm.bin:md5,d41d8cd98f00b204e9800998ecf8427e",
+                        "plink_simulated_ldms.grm.id:md5,d41d8cd98f00b204e9800998ecf8427e"
+                    ]
+                ]
+            ],
+            {
+                "versions_gcta": [
+                    [
+                        "GCTA_ADDGRMS",
+                        "gcta",
+                        "1.94.1"
+                    ]
+                ]
+            }
+        ],
+        "meta": {
+            "nf-test": "0.9.3",
+            "nextflow": "25.10.4"
+        },
+        "timestamp": "2026-05-15T21:10:46.231316108"
+    },
+    "homo_sapiens popgen - merge dense GRMs from mgrm": {
+        "content": [
+            [
+                [
+                    {
+                        "id": "plink_simulated_ldms"
+                    },
+                    [
+                        "plink_simulated_ldms.grm.N.bin:md5,804f8e1799c8b2d4d3df1b52a2a463c6",
+                        "plink_simulated_ldms.grm.bin:md5,850235911329bf9ab68f03e25bbc1ef1",
+                        "plink_simulated_ldms.grm.id:md5,4f9aa36c44a417ff6d7caa9841e66ad9"
+                    ]
+                ]
+            ],
+            {
+                "versions_gcta": [
+                    [
+                        "GCTA_ADDGRMS",
+                        "gcta",
+                        "1.94.1"
+                    ]
+                ]
+            }
+        ],
+        "meta": {
+            "nf-test": "0.9.3",
+            "nextflow": "25.10.4"
+        },
+        "timestamp": "2026-05-15T22:52:43.953267272"
+    }
+}
\ No newline at end of file