Crowned champions for the first time in April, Leverkusen beat Augsburg 2-1 on Saturday -- their 34th and final match of the league season.
Alonso's men, who have a Europa League final against Atalanta and a German Cup final versus Kaiserslautern to come this week, have not lost in any competition this season -- a run of 51 games.
"I'm proud to be part of this team. After all these years, I'm really happy to finally be able to achieve something here," said Tah who joined Leverkusen in 2015.
"It feels extremely good. Xabi Alonso is an outstanding coach both tactically and personally. That's why we are so successful".
Former Real Madrid, Liverpool and Bayern Munich star Alonso, however, played down his role.
"Good players make good coaches. I have an outstanding squad here. We had a good atmosphere every day. I'm very grateful to be able to work with such players," said the Spaniard.
Alonso said his side could celebrate into the night on Saturday, but would need to re-focus with a potential treble at stake.
"Today we'll celebrate, but from Monday at the latest we'll go full throttle for Wednesday. We have a clear plan."
Alonso, who joined Leverkusen in October 2022 with the club hovering near the relegation zone, ran to celebrate with the club's ultras after the match, leaping into the stands.
"Everything was safe with the fans up there, I can still jump well," Alonso said.
"It's not just the players, but also their families, the fans: we're all connected. We deserve this title so much, we stayed undefeated. It's a huge joy for us."
