diff --git a/Library/Homebrew/cmd/postinstall.rb b/Library/Homebrew/cmd/postinstall.rb
index 93ed03f059..967bba4695 100644
--- a/Library/Homebrew/cmd/postinstall.rb
+++ b/Library/Homebrew/cmd/postinstall.rb
@@ -24,8 +24,11 @@ module Homebrew
 
     args.named.to_resolved_formulae.each do |f|
       ohai "Postinstalling #{f}"
-      fi = FormulaInstaller.new(f, **{ debug: args.debug?, quiet: args.quiet?, verbose: args.verbose? }.compact)
-      fi.post_install
+      f.install_etc_var
+      if f.post_install_defined?
+        fi = FormulaInstaller.new(f, **{ debug: args.debug?, quiet: args.quiet?, verbose: args.verbose? }.compact)
+        fi.post_install
+      end
     end
   end
 end
diff --git a/Library/Homebrew/formula.rb b/Library/Homebrew/formula.rb
index 2e3ee7e9d2..d85768060b 100644
--- a/Library/Homebrew/formula.rb
+++ b/Library/Homebrew/formula.rb
@@ -1142,6 +1142,23 @@ class Formula
   sig { overridable.void }
   def post_install; end
 
+  # @private
+  sig { returns(T::Boolean) }
+  def post_install_defined?
+    method(:post_install).owner != Formula
+  end
+
+  # @private
+  sig { void }
+  def install_etc_var
+    etc_var_dirs = [bottle_prefix/"etc", bottle_prefix/"var"]
+    Find.find(*etc_var_dirs.select(&:directory?)) do |path|
+      path = Pathname.new(path)
+      path.extend(InstallRenamed)
+      path.cp_path_sub(bottle_prefix, HOMEBREW_PREFIX)
+    end
+  end
+
   # @private
   sig { void }
   def run_post_install
@@ -1164,13 +1181,6 @@ class Formula
         ENV.clear_sensitive_environment!
         ENV.activate_extensions!
 
-        etc_var_dirs = [bottle_prefix/"etc", bottle_prefix/"var"]
-        Find.find(*etc_var_dirs.select(&:directory?)) do |path|
-          path = Pathname.new(path)
-          path.extend(InstallRenamed)
-          path.cp_path_sub(bottle_prefix, HOMEBREW_PREFIX)
-        end
-
         with_logging("post_install") do
           post_install
         end
@@ -2174,6 +2184,7 @@ class Formula
       "disabled"                 => disabled?,
       "disable_date"             => disable_date,
       "disable_reason"           => disable_reason,
+      "post_install_defined"     => post_install_defined?,
       "service"                  => (service.serialize if service?),
       "tap_git_head"             => tap_git_head,
       "ruby_source_path"         => ruby_source_path,
diff --git a/Library/Homebrew/formula_installer.rb b/Library/Homebrew/formula_installer.rb
index d3d2833fb8..e22fac868e 100644
--- a/Library/Homebrew/formula_installer.rb
+++ b/Library/Homebrew/formula_installer.rb
@@ -796,7 +796,8 @@ on_request: installed_on_request?, options: options)
       puts "You can run it manually using:"
       puts "  brew postinstall #{formula.full_name}"
     else
-      post_install
+      formula.install_etc_var
+      post_install if formula.post_install_defined?
     end
 
     keg.prepare_debug_symbols if debug_symbols?
diff --git a/Library/Homebrew/formulary.rb b/Library/Homebrew/formulary.rb
index 1f072fe56e..20a53bd0c7 100644
--- a/Library/Homebrew/formulary.rb
+++ b/Library/Homebrew/formulary.rb
@@ -268,6 +268,12 @@ module Formulary
         raise "Cannot build from source from abstract formula."
       end
 
+      @post_install_defined_boolean = json_formula["post_install_defined"]
+      @post_install_defined_boolean = true if @post_install_defined_boolean.nil? # Backwards compatibility
+      def post_install_defined?
+        self.class.instance_variable_get(:@post_install_defined_boolean)
+      end
+
       if (service_hash = json_formula["service"].presence)
         service_hash = Homebrew::Service.deserialize(service_hash)
         service do
diff --git a/Library/Homebrew/test/formula_spec.rb b/Library/Homebrew/test/formula_spec.rb
index 82f525bd84..95c39ac23e 100644
--- a/Library/Homebrew/test/formula_spec.rb
+++ b/Library/Homebrew/test/formula_spec.rb
@@ -13,6 +13,7 @@ describe Formula do
   alias_matcher :have_changed_alias, :be_alias_changed
 
   alias_matcher :have_option_defined, :be_option_defined
+  alias_matcher :have_post_install_defined, :be_post_install_defined
   alias_matcher :have_test_defined, :be_test_defined
   alias_matcher :pour_bottle, :be_pour_bottle
 
@@ -636,6 +637,23 @@ describe Formula do
     expect(f.desc).to eq("a formula")
   end
 
+  specify "#post_install_defined?" do
+    f1 = formula do
+      url "foo-1.0"
+
+      def post_install
+        # do nothing
+      end
+    end
+
+    f2 = formula do
+      url "foo-1.0"
+    end
+
+    expect(f1).to have_post_install_defined
+    expect(f2).not_to have_post_install_defined
+  end
+
   specify "test fixtures" do
     f1 = formula do
       url "foo-1.0"