The 21-year-old Welshman's deal will keep him at Meadow Lane until January and he is expected to feature in the Magpies' pre-season friendly with Halifax on Saturday.
The move comes less than a month after Cassidy signed a new one-year deal with Wovles.
At the time, his manager Kenny Jackett told the club's website: "Cassidy is somebody that League One clubs will take and we can get a good loan so we feel in terms of making that decision (to extend his contract) is quite an easy one."
Cassidy came through the ranks at Wolves and he has had three separate half-season loan spells with Tranmere in the past three seasons.
He started the last campaign heavily involved at Wolves, then in Sky Bet League One, and played 18 times before moving to the Wirral club for the latter half of the season, where he made 19 more appearances.
Cassidy has scored 17 professional goals, with his best tally coming in the 2012/13 season when he fired 11 goals for Tranmere in 26 games.
The 6ft 2in striker becomes Notts manager Shaun Derry's 10th summer signing and the second from Wolves, after Zeli Ismail was also loaned to the Magpies by the Molineux side.