The deal for the 26-year-old means Jose Riga has a senior keeper in his squad for the first time since taking charge at Bloomfield Road in June - and with just four days until the Championship season begins.
Matt Gilks, Mark Halstead and Tony Warner all reached the end of their contracts over the summer, and Blackpool have an option to make Lewis' loan permanent at a later date.
Lewis made an England squad as a 20-year-old Peterborough player in 2008 but has played understudy to David Marshall at Cardiff for the past two years, making just five appearances.
He said: "I was sat at Cardiff for the last two seasons behind David Marshall, who has done outstanding.
"I've had to make do with a couple of cup games and the odd league game here and there, and it was pretty frustrating really. I knew that I had to get out and play some regular football.
"It's great to come to a club like Blackpool and hopefully make the No 1 position my own."