How can I put an HTML tag inside a <label> in Ruby on Rails?

Label accepts a block, so you can do this:

<%= f.label :test do %>
   <span>*</span>
<%end%>

You need to tell rails that the string you're giving it does not need to be sanitized. Use String#html_safe.

<%= f.label :test, "test<span>*</span>".html_safe %>