Michael Phelps says he's coming out of retirement only because he wants to get back into shape.  

The most decorated Olympian of all time will compete today at a meet in Mesa, Arizona.

Phelps says he missed competing and wants to get his body back into shape after gaining around 30 pounds.  

He wouldn't say if he'll compete at the 2016 Summer Olympics in Rio.  

He said he'll attend the Olympics in Brazil but only time will tell whether he's in the stands or in the pool.

Phelps hasn't swam competitively since the 2012 Olympics in London.  He's won 22 Olympic medals including 18 gold.