The left-hander was activated from the 15-day disabled list on Friday and rejoined the Mariners prior to their game against the Indians at Safeco Field.
The 27-year-old Australian native had been on the DL since July 28 with a lower back strain. He made five rehab starts for Triple-A Tacoma, including one Thursday night in the final home game of the Rainiers' season.
Rowland-Smith surrendered one earned run in seven innings against Colorado Springs and ended his prolonged rehab stint 2-4 with a 5.11 ERA.
"I had an idea I might [get activated] because there are only four games left and I pitched on Thursday," he said. "I was hoping, but wasn't 100 percent sure until they told me last night, so I didn't have to get up at 5 a.m. like the rest of the guys."
The Rainiers traveled to Fresno, Calif., for the last four games of the Pacific Coast League regular season, clinging to a 1 1/2-game lead in the North.
Rowland-Smith returned to the Mariners much more confident than when he left in July, packing with him a 1-10 record and 6.69 ERA in 22 games, including 19 starts.
"I saw him when he first walked into the [Tacoma] clubhouse in Iowa, and he wasn't as confident as I'd seen him before," said interim manager Daren Brown. "That kind of bothered me. That's a big tool he's always had.
"I've said it before, that we can grade out fastballs and grade out offspeed pitches, but what a guy has inside, as far as wanting to compete and wanting to go out and beat the other guy, that's tough to grade that out.
"He's always been a very competitive guy, and in talking to him today, I think he's gotten some of that back. I'm looking forward to getting him out there in four or five days."
Rowland-Smith will pitch out of the bullpen for the time being, possibly getting some starts during the final three weeks of the season.
"We've got some off-days coming up and we can work around those," Brown said. "But if we do need a spot start, maybe back a guy up and give [another starter] an extra day, he gives us a little of that."
Rowland-Smith's preivous outing with Seattle was July 27 against the Cubs at Safeco Field, where he surrendered 11 runs on 11 hits in five innings.