Luis Castillo is heading west…

The Seattle Mariners, looking to end a postseason drought that stretches to 2001, acquired the best arm on the trade market on Friday night in a deal for the 29-year-old Dominican professional baseball pitcher from the Cincinnati Reds.

“He’s one of the best pitchers in the game — he’s really established himself as a dominant starter,” Mariners manager Scott Servais said. “We’ve got a chance to do something really big here this year. You have to step out and take a chance once in a while if you ultimately want to get the reward, take a little risk. Dominant starting pitcher, and I’m anxious to meet him.”

Minor leaguers Noelvi Marte, Levi Stoudt, Edwin Arroyo and Andrew Moore are headed to Cincinnati, the teams announced.

Marte, a shortstop, is the highest rated of the group, with ESPN’s Kiley McDaniel ranking him the 12th-best prospect in baseball ahead of the season.

“We felt this was the best return we could get for Luis,” Reds general manager Nick Krall said.

The Mariners, led by star rookie Julio Rodriguez, are 54-47, 12 games behind the first-place Houston Astros in the AL West. They are in the second of three AL wild-card spots, a half-game in front of Tampa Bay (53-47) and two games ahead of Cleveland (51-48). Seattle won its final 14 games before the MLB All-Star break, one short of the longest winning streak in team history and the best run by any club heading into the break since 1933. But the Mariners are 3-5 since, following an 11-1 loss at Houston on Friday night.

Now, they’ve added the best available arm ahead of the Tuesday trade deadline, one who will join an already-solid rotation that includes Robbie Ray and Logan Gilbert.

It’s a massive pickup for a Mariners organization that has not reached the postseason since 2001, the longest active drought among the four major North American pro sports leagues (MLB, NFL, NBA, NHL).

Castillo is 4-4 with a 2.86 ERA in 14 starts for the Reds this season, striking out 90 and walking 28 in 85 innings. His fastball averages 97 mph. He has a career 3.62 ERA in six seasons, all with Cincinnati.

“It has been a beautiful experience,” Castillo said through an interpreter. “To have my name in the fans’ mouth and having them cheer me on, it is something I will treasure forever.”

Castillo won’t be eligible for free agency until after the 2023 season, so the Mariners would have him for at least the rest of this year and all of next.

Castillo has bounced back from a career-worst season in 2021, when he lost 16 games and walked a league-high 75 batters. His changeup has been his best pitch for most of his career, but this year, he is actually throwing his four-seam fastball more, and doing so with a lot of success. Opponents have whiffed on 39% of their swings against his four-seamer, the highest rate of Castillo’s career and the highest of any pitcher in the majors (minimum 300 four-seamers thrown).

Castillo has a 1.38 ERA in four starts against the AL this year, second to the Los Angeles Dodgers’ Clayton Kershaw (0.69) among qualified pitchers.

The Reds have been moving veteran players for prospects since the end of the lockout earlier this year. In March, they traded Eugenio Suarez and Jesse Winker to the Mariners, and Thursday, they traded outfielder Tyler Naquin and reliever Phillip Diehl to the New York Mets.

They’re getting a haul from Seattle in this trade, with Marte and Arroyo ranked among the team’s top 10 prospects ahead of the season by ESPN’s McDaniel.

Marte, 20, has spent the year for High-A Everett, hitting .275 with 15 home runs and a team-best 62 runs scored and 55 RBIs to go along with 13 steals entering Friday.

McDaniel ranked Arroyo, also a shortstop, as Seattle’s eighth-best prospect entering the season. The 18-year-old has spent the season at Single-A Modesto, hitting .316 with 76 runs, 67 RBIs, 13 home runs and 21 steals.

Stoudt, a 24-year-old right-hander and a third-round draft pick in 2019, is 6-6 with a 5.28 ERA in 18 starts for Double-A Arkansas this season.

And Moore, 22, has a 1.95 ERA in 25 relief appearances at Modesto this season.

“Noelvi Marte has a chance to be an impact middle of the order bat who can play shortstop,” Krall said. “Edwin Arroyo same thing — he has power/speed combo, who can play shortstop. Levi Stoudt has a chance to be a major league starter. Andrew Moore just started to pitch. He was drafted last year. He has explosive, explosive stuff. His fastball is up to 102 with a plus slider.”

Cincinnati infielder Brandon Drury also could be moved before the Tuesday deadline, along with several Reds relievers.