The 20-year-old Spanish midfielder rejected a new deal to remain at Emirates Stadium at the end of last season and officially began his Atletico career on Monday after making just six appearances in four seasons with the Gunners.
“I didn't have to think twice about leaving,” Merida told Spanish daily AS. "I'm an Atletico fan and I feel enormously proud to be here.
By his own admission, Merida "struggled to fit in" at Arsenal, and was consequently loaned out to Real Sociedad in 2007/08 after signing his first professional deal on his 17th birthday.
"Atletico is a big club and so is Arsenal, but there were some circumstances there, particularly on the sporting side, which I wasn't happy with and I couldn't fit in.
"I made Arsene Wenger aware of that and as soon as I knew Atletico were interested, I didn't think twice."
Merida rose through the ranks at Barcelona before following in Cesc Fabregas' footsteps in swapping the Catalonia for North London.
But the left-footed youngster struggled to make an impression at Arsenal before agreeing a deal at the Vicente Calderon.
By James Martini
‘He instantly popped into our ratings as one of the best U21 midfielders in Europe. The impressive thing is Slot has made him into more of a no.6' Inside Ryan Gravenberch's transformation at Liverpool
‘I don’t think Liverpool would look at Ollie Watkins, a striker isn’t a pressing issue for them – it’s Arsenal who need one’ Former Reds star explains why his old club don’t need an out-and-out forward this summer
