Belgium's Remco Evenepoel (23) retained the overall leader's red jersey after finishing safely in the pack having picked up useful time bonuses on the last intermediate sprint.
Groves strengthened his hold on the green jersey after following up on Tuesday's effort.
"It’s a fantastic feeling, the second win of this Vuelta, and two in a row, it’s a great feeling especially doing it with the green jersey," Groves said.
"At the end, it was a pretty hectic final today, similar to yesterday but without climbing. The wind and roundabouts made it quite nervous but my team was fantastic again."
Uruguay's Eric Fagundez (Burgos BH) had spent most of an undulating stage in the province of Castellon out in front alone after making an early move that did not tempt any other riders.
Having pushed his advantage to more than five minutes Fagundez was eventually caught on the day's only categorised climb by Argentina's Eduardo Sepulveda (Lotto Dstny) around 40 kilometres from the finish - Sepulveda adding to his lead in the climbers category, before being swallowed up by the peloton.
A crash 3km from the finish raised the tension but once Groves was positioned into the lead by his team mates he held on despite the efforts of Italian Ganna who finished like an express train.