Showunmi has signed a two-year deal at Meadow Lane after passing a medical on Tuesday afternoon and the experienced striker becomes the Magpies' first summer signing.
The 30-year-old, a Nigeria international, began his career at Luton in 2003, scoring 15 goals and helping the Hatters win the League One title in 2005.
A year later he joined Bristol City and scored 10 goals in 28 starts during his first season as the Robins gained promotion to the Championship.
Showunmi also had spells with Sheffield Wednesday, Leeds and Falkirk prior to signing for Tranmere in 2010. Showunmi scored 17 goals in 79 appearances for Rovers.