diff --git a/lib/ransack/nodes/condition.rb b/lib/ransack/nodes/condition.rb
index 735bd183..2b07b859 100644
--- a/lib/ransack/nodes/condition.rb
+++ b/lib/ransack/nodes/condition.rb
@@ -235,11 +235,21 @@ def formatted_values_for_attribute(attr)
             val = attr.ransacker.formatter.call(val)
           end
           val = predicate.format(val)
+          
+          # Handle case where formatter for IN/NOT IN predicates returns a string with comma-separated values
+          if predicate.wants_array && val.is_a?(String) && val.include?("','")
+            val = val.split("','")
+          end
+          
           if val.is_a?(String) && val.include?('%')
             val = Arel::Nodes::Quoted.new(val)
           end
           val
         end
+        
+        # Flatten the array in case any formatters returned arrays
+        formatted = formatted.flatten if predicate.wants_array
+        
         if predicate.wants_array
           formatted
         else
diff --git a/spec/ransack/predicate_spec.rb b/spec/ransack/predicate_spec.rb
index cf84e200..8377456e 100644
--- a/spec/ransack/predicate_spec.rb
+++ b/spec/ransack/predicate_spec.rb
@@ -473,6 +473,25 @@ module Ransack
           expect(@s.result.to_sql).to match /#{field} NOT IN \('a', 'b'\)/
         end
       end
+
+      describe "with 'in' arel predicate and string formatter" do
+        before do
+          Ransack.configure do |config|
+            config.add_predicate 'in_list',
+              arel_predicate: 'in',
+              formatter: ->(v) { v&.split(';').join("','") }
+          end
+        end
+
+        it 'generates correct IN clause without extra quotes' do
+          @s.name_in_list = 'test;othertest'
+          field = "#{quote_table_name("people")}.#{quote_column_name("name")}"
+          # Should generate: WHERE people.name IN ('test', 'othertest')
+          # Not: WHERE people.name IN ('test'',''othertest')
+          expect(@s.result.to_sql).to match /#{field} IN \('test', 'othertest'\)/
+          expect(@s.result.to_sql).not_to match /#{field} IN \('test'',''othertest'\)/
+        end
+      end
     end
 
     private