The Texans agreed to terms with Clark on Tuesday and made it official on Wednesday.
Houston needed to add depth at the position after Brian Cushing was suspended for the first four games of the season for violating the league's drug policy.
Clark played for the Texans in 2007 and spent the last two seasons with the New York Giants. He appeared in each game, started 11 and finished with 53 tackles last year.
The 33-year-old has been in the league for 10 years and his best seasons came with the Oakland Raiders, where he finished with more than 110 tackles in both 2004 and 2005.