In March 2015, Viljoen was named in an extended South Africa Under-20 training group as part of their preparation for the 2015 World Rugby Under 20 Championship. He didn't feature in a friendly match against a Varsity Cup Dream Team&lt;ref name="Varsity Cup XV 24-31 South Africa U/20"&gt;&lt;/ref&gt; or their two-match tour of Argentina, but was included in the final squad for the 2015 World Rugby Under 20 Championship.