After serving the youth system, Adler joined the B team in 2002.
He made eight appearances for the B team in the 2002–03 season.
At the beginning of the 2003–04 season he became part of the club's first team, which played in the Bundesliga.
Nevertheless, he didn't make an appearance and continued to play for the second team.
He also failed to make any first team appearances during the 2004–05 and 2005–06 seasons.
However, in the 2005–06 Regionalliga season, he made 19 appearances for the B team.
In May 2006 he sustained a rib craze, which led to a surgery in which his rib was strengthened.
Because of his injury Adler had to pause until February 2007.

He made his Bundesliga debut in dramatic fashion on 25 February 2007 against league leaders Schalke 04.
With starting goalkeeper Hans-Jörg Butt serving a suspension following a red card, Adler saved ten shots on goal, repelling Schalke's repeated attacks.
A breakaway goal by Stefan Kießling in the 85th minute gave Leverkusen a 1–0 upset victory.
Leverkusen manager Michael Skibbe praised Adler's "world-class saves".
He stopped Schalke's record of 13 games unbeaten, Stuttgart's record of five games unbeaten and Hamburger SV four-game winning spree with a saved penalty kick against Rafael van der Vaart.
He ended up making 15 appearances in all competitions for the first team and two appearances for the B team.
On 8 March 2008 Adler made his first international appearance, losing the UEFA Cup round of last 16 first leg against RC Lens.
He finished the 2007–08 season with 44 appearances in all competitions for the first team.

In the following years, Adler continued to be the first choice goalkeeper until newly signed Bernd Leno took his place due to injuries.
He had made 37 appearances in the 2008–09 season, 33 appearances in 2009–10 season, and 44 appearances in the 2010–11 season.
Previously, Leverkusen wanted to renew his contract but Adler rejected those offers.
In summer 2012, Adler left the club for a free transfer after twelve years with Leverkusen.
He had only made 2 B team appearances during the 2011–12 season.
After serving the youth system, Adler joined the B team in 2002. He made eight appearances for the B team in the 2002–03 season. At the beginning of the 2003–04 season he became part of the club's first team, which played in the Bundesliga. Nevertheless, he didn't make an appearance and continued to play for the second team. He also failed to make any first team appearances during the 2004–05 and 2005–06 seasons. However, in the 2005–06 Regionalliga season, he made 19 appearances for the B team. In May 2006 he sustained a rib craze, which led to a surgery in which his rib was strengthened. Because of his injury Adler had to pause until February 2007.
He made his Bundesliga debut in dramatic fashion on 25 February 2007 against league leaders Schalke 04. With starting goalkeeper Hans-Jörg Butt serving a suspension following a red card, Adler saved ten shots on goal, repelling Schalke's repeated attacks. A breakaway goal by Stefan Kießling in the 85th minute gave Leverkusen a 1–0 upset victory. Leverkusen manager Michael Skibbe praised Adler's "world-class saves". He stopped Schalke's record of 13 games unbeaten, Stuttgart's record of five games unbeaten and Hamburger SV four-game winning spree with a saved penalty kick against Rafael van der Vaart. He ended up making 15 appearances in all competitions for the first team and two appearances for the B team. On 8 March 2008 Adler made his first international appearance, losing the UEFA Cup round of last 16 first leg against RC Lens. He finished the 2007–08 season with 44 appearances in all competitions for the first team.
In the following years, Adler continued to be the first choice goalkeeper until newly signed Bernd Leno took his place due to injuries. He had made 37 appearances in the 2008–09 season, 33 appearances in 2009–10 season, and 44 appearances in the 2010–11 season. Previously, Leverkusen wanted to renew his contract but Adler rejected those offers. In summer 2012, Adler left the club for a free transfer after twelve years with Leverkusen. He had only made 2 B team appearances during the 2011–12 season.
René Adler