The young Czech will start his stay with Cologne, who were promoted to the Bundesliga last season, on July 1.
This will be his third loan stint, having spent the 2011-12 and 2012-13 seasons in the Netherlands with Vitesse Arnhem.
Kalas returned to Chelsea last season and made his debut as a late substitute in the 2-0 Capital One Cup win over Arsenal.
He was a regular on the substitutes' bench towards the end of the season, making his first Premier League start in the impressive 2-0 win over Liverpool that dealt a huge blow to the Anfield club's hopes of winning the title.
"I'd like to play regularly to return to Chelsea even stronger. The Bundesliga in a famous team like this will be a great test for me," Kalas said.
Cologne were relegated to the second division at the end of the 2010-11 season but secured the title last season, beating SC Paderborn by six points.