Episode 126 dates back to 1965 and was found in an unmarked tin in West London.
The show ran from 1964 to 1988.
After finding two reels of film from 1973, researchers discovered a third can, labelled only BM TR 2096.
Fans of the show can watch the episode for free from Friday morning at www.itv.com/crossroads.
They will also be able to pre-order the DVD of the other two 'missing' episodes from 1973 as well as the 1965 show.
At the time, the only comparable serials were Granada's Coronation Street and the BBC's Compact.
A five nights per week soap was a completely new idea at this time.
Crossroads was first broadcast on November 2, 1964.
It ran to 4,510 episodes before it was axed.
It was set in the fictitious Midlands village of Kings Oak, just off the motorway.
The soap was about the lives of the staff and guests at the Crossroads motel.
It was opened by the shrewd Meg Richardson (Noele Gordon) following the death of her husband when motorway construction came through part of her land.
Noele Gordon was the original 'Queen of Soaps.
