diff --git a/app/models/case_log.rb b/app/models/case_log.rb
index 46d766ccc..9a84c633e 100644
--- a/app/models/case_log.rb
+++ b/app/models/case_log.rb
@@ -15,6 +15,16 @@ class CaseLogValidator < ActiveModel::Validator
     end
   end
 
+  def validate_other_reason_for_leaving_last_settled_home(record)
+    if record.reason_for_leaving_last_settled_home == "Other" && record.other_reason_for_leaving_last_settled_home.blank?
+      record.errors.add :other_reason_for_leaving_last_settled_home, "If reason for leaving settled home is other then the other reason must be provided"
+    end
+
+    if record.reason_for_leaving_last_settled_home != "Other" && record.other_reason_for_leaving_last_settled_home.present?
+      record.errors.add :other_reason_for_leaving_last_settled_home, "The other reason must not be provided if the reason for leaving settled home was not other"
+    end
+  end
+
   def validate(record)
     # If we've come from the form UI we only want to validate the specific fields
     # that have just been submitted. If we're submitting a log via API or Bulk Upload
@@ -87,6 +97,14 @@ private
   end
 
   def mandatory_fields
-    attributes.except(*AUTOGENERATED_FIELDS)
+    required = attributes.except(*AUTOGENERATED_FIELDS)
+
+    dynamically_not_required = []
+
+    if reason_for_leaving_last_settled_home != "Other"
+      dynamically_not_required << "other_reason_for_leaving_last_settled_home"
+    end
+
+    required.delete_if { |key, _value| dynamically_not_required.include?(key) }
   end
 end
diff --git a/spec/fixtures/complete_case_log.json b/spec/fixtures/complete_case_log.json
index 11f073714..da2e0da20 100644
--- a/spec/fixtures/complete_case_log.json
+++ b/spec/fixtures/complete_case_log.json
@@ -89,7 +89,7 @@
       "chr_letting": false,
       "cap_letting": false,
       "outstanding_rent_or_charges": 25,
-      "other_reason_for_leaving_last_settled_home": "Other reason",
+      "other_reason_for_leaving_last_settled_home": null,
       "accessibility_requirements_fully_wheelchair_accessible_housing": true,
       "accessibility_requirements_wheelchair_access_to_essential_rooms": false,
       "accessibility_requirements_level_access_housing": false,
diff --git a/spec/models/case_log_spec.rb b/spec/models/case_log_spec.rb
index 6b0805250..dff400389 100644
--- a/spec/models/case_log_spec.rb
+++ b/spec/models/case_log_spec.rb
@@ -25,6 +25,22 @@ RSpec.describe Form, type: :model do
     it "validates number of relets is over 0" do
       expect { CaseLog.create!(property_number_of_times_relet: 0) }.to raise_error(ActiveRecord::RecordInvalid)
     end
+
+    context "other reason for leaving last settled home validation" do
+      it "must be provided if main reason for leaving last settled home was given as other" do
+        expect {
+          CaseLog.create!(reason_for_leaving_last_settled_home: "Other",
+                          other_reason_for_leaving_last_settled_home: nil)
+        }.to raise_error(ActiveRecord::RecordInvalid)
+      end
+
+      it "must not be provided if the main reason for leaving settled home is not other" do
+        expect {
+          CaseLog.create!(reason_for_leaving_last_settled_home: "Repossession",
+                          other_reason_for_leaving_last_settled_home: "the other reason provided")
+        }.to raise_error(ActiveRecord::RecordInvalid)
+      end
+    end
   end
 
   describe "status" do