The win maintained GB’s unbeaten run in the Olympic campaign, having won all five pool games, with one more win giving them a shot of gold in Friday’s final.
First-half goals from Twigg, Owsley and Helen Richardson-Walsh gave Britain a solid advantage at the break, with Spain unable to break down Team GB’s defence.
A Spanish goal in the fourth quarter was little more than a consolation and Owsley said the side’s first half performance paved the way for their progression.
“We got on the scoreboard pretty early, which I always a big thing in massive games like this,” she said.
“It was a bit more of a nervy second half, they got one back, but we’ve shown we can grind out games now.
“We’re just taking it a game at the time. It’s easy to think that if we win this, if you win that…You’re not going to get to the final if you don’t take each game as it comes,” she said.
Twigg opened the scoring after just seven minutes of the game from a penalty corner before Richardson-Walsh doubled the lead four minutes later.
A third goal from Owsley towards the end of the second period looked to put the game out of Spain’s reach before half-time.
Britain’s stout defence continued after half-time, with Spain’s first real chance coming with a minute left in the third period, but the post maintained GB’s clean sheet. The Spanish did get the ball in the net through Georgina Oliva with seven minutes remaining, but the comeback was not to be.
“I’m very proud. Our defence was strong when it needed to be and we were just superb in the first half,” said coach Danny Kerry.
“New Zealand are a very fast side. They’ve got a lot of pace and a lot of good goalscorers and it will be a hell of a game.”
A win against New Zealand on Wednesday would guarantee Britain’s women a silver medal, which would equate to their best ever Olympic performance, having finished at both London 2012 and Barcelona 1992.