Ivan "Ivica" Grnja, also known as John Grnja (born 26 April 1949) is a Croatian football manager and former professional player. He was born in Darda.
Grnja played in the North American Soccer League for the Toronto Metros-Croatia and the Tampa Bay Rowdies.
Grnja has managed Croatian club side NK Osijek on two occasions - in 1991, and between 1993 and 1994.
Grnja also managed the Croatia under-21 side in 2005, and the under-19 side in 2008.