"I did everything I could in Latvia. I'm not there anymore," he wrote on Facebook.
Girss claimed on Facebook that after being held for a few minutes on the border, he was allowed to continue to his "beloved Russia".
He said it remains an open question whether he'll change his citizenship. He burned his Latvian passport on video in 2015 in protest against what he perceived as unfair treatment of Latvia's Russian-speaking minority.
Along with people like Vladimir Linderman and Jevgeny Osipov, Girss was one of the most noticeable pro-Russia activists in Latvia.
He was among the organizers of the failed 2012 referendum on making Russian an official state language. In 2013 the Latvian Security Police opened a case against Linderman, Osipov and Girss over allegedly spreading anti-constitutional ideas of autonomy for Latgale, an eastern region of Latvia.