The Italy Test star, who joined Tigers last year, beat his Leicester team-mate Shane Jennings for top spot with Bristol lock Roy Winters third.
Wasps' England flanker Tom Rees took young player of the season honours, edging out club colleagues James Haskell and Dominic Waldouck, while Pat Howard won the director of rugby award with his Leicester side just two wins away from an unprecedented trophy treble of Premiership title, Heineken Cup and EDF Energy Cup.
Castrogiovanni, 25, becomes the second overseas player to land the top award, following Gloucester's South African flanker Jake Boer in 2003.
"In his first season in the Guinness Premiership, Martin has been a rock for Leicester in what may yet be the most successful season in the club's history," said Premier Rugby's rugby manager Phil Winstanley.
"His enormous contribution was recognised by the voting panel in a position in the front-row that is not widely recognised."
As for the young player category award, Winstanley added: "At one stage I think we had a three-way tie for this award.
"In the end, the panel felt in Tom Rees we had a player who had moved on from being a support player in his club squad to be first choice with his club and an England international."
The Premiership dream team, meanwhile, contained four players from title play-off candidates Bristol - Winters, prop Darren Crompton, number eight Dan Ward-Smith and wing David Lemi.
Guinness Premiership player of the season - Martin Castrogiovanni (Leicester); runner-up - Shane Jennings (Leicester); third - Roy Winters (Bristol).
Land Rover Discovery of the season - Tom Rees (Wasps); runner-up - James Haskell (Wasps); third - Dominic Waldouck (Wasps).
O2 director of rugby of the season - Pat Howard (Leicester).
MBNA try of the season - Mark Foster (Gloucester) versus Harlequins.
Gilbert golden boot award - Glen Jackson (Saracens).
EDF Energy community player of the season - Matt Perry (Bath).
Gillette Power award - Alex Brown (Gloucester).
The Times team performance of the season - Bristol for Bristol v Gloucester (November 24).
Sky Sports dream team - M Brown (Harlequins); D Strettle (Harlequins), S Mapusua (London Irish), T Flood (Newcastle), D Lemi (Bristol); G Jackson (Saracens), H Ellis (Leicester); M Castrogiovanni (Leicester), G Chuter (Leicester), D Crompton (Bristol), R Winters (Bristol), M Bortolami (Gloucester, capt), J M Fernandez Lobbe (Sale Sharks), A Hazell (Gloucester), D Ward-Smith (Bristol).