diff --git a/common.mk b/common.mk
index de7b553dee..3000a49319 100644
--- a/common.mk
+++ b/common.mk
@@ -70,7 +70,6 @@ HTMLOUT       = $(EXTOUT)/html
 CAPIOUT       = doc/capi
 INSTALL_DOC_OPTS = --rdoc-output="$(RDOCOUT)" --html-output="$(HTMLOUT)"
 RDOC_GEN_OPTS = --no-force-update \
-	--title "Documentation for Ruby $(RUBY_API_VERSION)" \
 	$(empty)
 
 INITOBJS      = dmyext.$(OBJEXT) dmyenc.$(OBJEXT)
diff --git a/tool/rdoc-srcdir b/tool/rdoc-srcdir
index 10c63caf9e..304deb67ce 100755
--- a/tool/rdoc-srcdir
+++ b/tool/rdoc-srcdir
@@ -9,6 +9,12 @@ invoked = Dir.pwd
 Dir.chdir(File.dirname(__dir__))
 
 options = RDoc::Options.load_options
+options.title = options.title.sub(/Ruby \K.*version/) {
+  File.read("include/ruby/version.h")
+    .scan(/^ *# *define +RUBY_API_VERSION_(MAJOR|MINOR) +(\d+)/)
+    .sort # "MAJOR" < "MINOR", fortunately
+    .to_h.values.join(".")
+}
 options.parse ARGV
 
 options.singleton_class.define_method(:finish) do