<s:optgroup/>
is your friend here.
From Struts2 JavaDoc:
org.apache.struts2.components
Class OptGroup
Create a optgroup component which needs to resides within a select tag.
This component is to be used within a Select component.
<s:select label = "My Selection" name = "mySelection" value = "%{'POPEYE'}" list = "%{#{'SUPERMAN':'Superman', 'SPIDERMAN':'spiderman'}}"> <s:optgroup label = "Adult" list = "%{#{'SOUTH_PARK':'South Park'}}" /> <s:optgroup label = "Japanese" list = "%{#{'POKEMON':'pokemon','DIGIMON':'digimon'}}" /> </s:select>