Yes, he was. Of course, most blacks were Republicans until the mid-late ‘60s.
If I remember right, MLK said he voted for Eisenhower in 1956.
His having been a “registered Republican” has been debunked (as per that Frances Rice article, posted here more times than I can count, which was full of inaccuracies). The only definitive evidence was his FATHER having been one until early 1960. Jr. was VERY anti-Conservative Republican, calling Goldwater a tool of Southern racists.
Also, most Northern Blacks became Democrats by the mid ‘30s, Southern Blacks not until the early ‘60s, but the latter mostly couldn’t vote, and when they registered en masse by 1964, they voted over 90% Democrat.
I read Congressman Adam Clayton Powell, Jr. voted for Ike in ‘56, but that was often confused for MLK, Jr., for whom I still don’t know if he voted for Ike or Stevenson. A majority of Blacks (again, mostly Northern) who could vote in ‘56 went with Stevenson.