As his contract ended this week, Witsel, a 36-year-old Belgian veteran, will be leaving the Madrid-based team.
During his time with Atletico, Witsel made 115 appearances and scored two goals under coach Diego Simeone.
Witsel expressed his gratitude to the club, his teammates, staff, and fans in a heartfelt message.
He shared that he had been privileged to experience intense and emotional moments while playing for Atletico, and he considered it an honour to defend the team's colours.
Witsel ended his message with a rallying cry for Atletico fans, saying "Aúpa Atleti!" which roughly translates to "Up Atletico!"