“I Can Only Imagine” can refer to both a song and a movie, and they were released at different times:

  • The song “I Can Only Imagine” by MercyMe was originally released in 1999 on their independent album and then widely released in 2001 on the album Almost There.
  • The film I Can Only Imagine (the Bart Millard/MercyMe biographical movie) was released in U.S. theaters on March 16, 2018.

If you meant the movie specifically, the most standard answer is: it was released on March 16, 2018.