Montero cleared waivers and was outrighted by the Astros to Triple-A Sugar Land on Saturday.
The Astros designated Montero for assignment Wednesday.
Montero signed a three-year, $34.5 million deal to return to the Astros on Saturday, Jeff Passan of ESPN.com reports.
Montero picked up the save with a scoreless ninth inning against the Rays on Sunday. He allowed one hit and two walks while striking out one.
Montero (5-2) earned the win Saturday against the Orioles. He allowed two runs on two hits while striking out one over one inning.
Montero earned the save Thursday against Oakland, striking out two in a perfect ninth inning in a 5-2 victory.
Montero recorded the save Sunday against Baltimore, allowing a run on three hits while striking out two in the ninth inning of a 3-1 victory.
Montero recorded a save against the Twins on Thursday by striking out one batter in a perfect inning of work.
Montero allowed a run on one hit and struck out two in one inning to earn the save in Tuesday's 9-7 win over the Royals.
Montero allowed one hit and struck out one across a scoreless seventh inning to earn a hold Sunday against the White Sox.