Czech rider Pavel Bittner and Belgium's Vito Braet came second and third in a mass sprint finish on very wet streets, which hosted a Vuelta finish for the first time in over 20 years and a stage departure for the first time in a decade.
The large group of sprinters who crossed the finish line included overall leader, Australian Ben O'Connor, and his main challenger, Slovenian Primoz Roglic.
O'Connor maintained his five-second lead in the overall standings.
"It was quite a tough day. It started dry and then the finish was wet so it made it quite dangerous as well," Groves told reporters.
"There are only a few sprint stages in this race and the rest are mountain stages so the intermediate riders really have to try on days like today."
A leading group of four riders followed each other in the mountains of northwestern Spain into the mostly flat final stretch but lost time as none of them wanted to attempt a breakaway.
The peloton cut into the lead as they passed through the city centre and caught up with the leaders in the last kilometre.
O'Connor praised his compatriot Groves, who also took over the green points jersey following Wout van Aert's abandonment after a crash on Tuesday.
"It's always good to see another fellow Aussie win," O'Connor said.
"It was a bit wet, not too hard, a little treacherous on the descents, but it wasn’t too complicated in the end."
