"DOUBLE INSTA FOR LOVE!!!!!!!! My heart is exploding!!" Gustin's longtime friend, Leela Rothenberg wrote alongside a photo of the couple celebrating.
A source confirms to ET that Gustin popped the question to Thoma on Saturday, after she moved into his Venice home on Friday. The couple were working out together on the beach, when she turned around to find Gustin down on one knee with the ring in hand.
Gustin might soon have two weddings to plan. Fans can't wait for the 27-year-old actor's character, Barry Allen, to wed Candice Patton's Iris on The Flash.
"When I knew that they were going to have another proposal, I think it was the sweetest thing," Patton gushed to ET last month. "And I think it's more WestAllen, this proposal. It's at home, it's very casual."
"Barry was kind of re-establishing his confidence in the relationship. Not letting his fear dictate things, but rather let his heart dictate things, which he's always done in the past," Gustin said. "So we're kind of back to true-form Barry, I think. As much as we can be with what he's dealing with right now."