diff --git a/CHANGES.rst b/CHANGES.rst
index 32083a48..cc551769 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -6,6 +6,7 @@ Version 0.9.4 (UNRELEASED)
 
 - Changes the REANA specification schema to use the ``draft 2020-12`` version of the JSON schema specification.
 - Changes validation of REANA specification to expose functions for loading workflow input parameters and workflow specifications.
+- Changes the validation schema of the REANA specification to make the ``environment`` property mandatory for the steps of serial workflows.
 - Fixes the mounting of CVMFS volumes for the REANA deployments that use non-default Kubernetes namespace.
 
 Version 0.9.3 (2023-09-26)
diff --git a/reana_commons/validation/schemas/reana_analysis_schema.json b/reana_commons/validation/schemas/reana_analysis_schema.json
index 65b9afa0..9bd671a1 100644
--- a/reana_commons/validation/schemas/reana_analysis_schema.json
+++ b/reana_commons/validation/schemas/reana_analysis_schema.json
@@ -108,6 +108,109 @@
           }
         }
       },
+      "oneOf": [
+        {
+          "properties": {
+            "type": {
+              "const": "serial"
+            },
+            "specification": {
+              "type": "object",
+              "title": "Serial workflow specification.",
+              "description": "Serial workflow specification.",
+              "additionalProperties": false,
+              "properties": {
+                "steps": {
+                  "type": "array",
+                  "title": "Serial workflow steps.",
+                  "description": "List of steps which represent the workflow.",
+                  "items": {
+                    "properties": {
+                      "name": {
+                        "type": "string",
+                        "title": "Step name."
+                      },
+                      "environment": {
+                        "type": "string",
+                        "title": "Image to be used by the container in which the step should be run."
+                      },
+                      "kubernetes_memory_limit": {
+                        "type": "string",
+                        "title": "Memory limit for the step container (e.g. 256Mi)."
+                      },
+                      "commands": {
+                        "type": "array",
+                        "title": "Step commands.",
+                        "description": "List of commands to be run in the step.",
+                        "items": {
+                          "type": "string",
+                          "title": "Command to be run."
+                        }
+                      }
+                    },
+                    "required": [
+                      "environment"
+                    ]
+                  }
+                }
+              }
+            }
+          }
+        },
+        {
+          "properties": {
+            "type": {
+              "const": "cwl"
+            },
+            "specification": {
+              "type": "object",
+              "title": "CWL workflow specification.",
+              "description": "CWL workflow specification.",
+              "properties": {
+                "cwlVersion": {
+                  "type": "string",
+                  "title": "CWL version.",
+                  "description": "CWL version to which the workflow was written for."
+                },
+                "class": {
+                  "type": "string",
+                  "title": "CWL class.",
+                  "description": "CWL class which represents the type of the workflow."
+                },
+                "inputs": {
+                  "type": "array",
+                  "title": "CWL workflow inputs.",
+                  "description": "CWL workflow inputs."
+                }
+              }
+            }
+          }
+        },
+        {
+          "properties": {
+            "type": {
+              "const": "snakemake"
+            },
+            "specification": {
+              "type": "object",
+              "title": "Snakemake workflow specification.",
+              "description": "Snakemake workflow specification."
+            }
+          }
+        },
+        {
+          "properties": {
+            "type": {
+              "const": "yadage"
+            },
+            "specification": {
+              "type": "object",
+              "title": "Yadage workflow specification.",
+              "description": "Yadage workflow specification."
+            }
+          }
+        }
+      ],
       "anyOf": [
         {
           "required": [
diff --git a/reana_commons/validation/utils.py b/reana_commons/validation/utils.py
index 6038e635..ae7f5fc5 100644
--- a/reana_commons/validation/utils.py
+++ b/reana_commons/validation/utils.py
@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 #
 # This file is part of REANA.
-# Copyright (C) 2022 CERN.
+# Copyright (C) 2022, 2023 CERN.
 #
 # REANA is free software; you can redistribute it and/or modify it
 # under the terms of the MIT License; see LICENSE file for more details.
@@ -77,6 +77,8 @@ def validate_reana_yaml(reana_yaml: Dict) -> Dict:
     """Validate REANA specification file according to jsonschema.
 
     :param reana_yaml: Dictionary which represents REANA specification file.
+    :returns: Dictionary of non-critical warnings, in the form of
+    {warning_key: [warning_value1, warning_value2, ...]}.
     :raises ValidationError: Given REANA spec file does not validate against
         REANA specification schema.
     """