Romania is not yet a member of the Schengen zone, but intends to join.
In the mean time holders of a multiple entry Schengen Visa do not need to apply for a separate Romania visa.
The Romanian ministry of foreign affairs mentions this:
Starting July 11, 2014, in keeping with the provisions of Decision No
565/2014/EU, Romania acknowledges as being equivalent to its own
national visas, for transit and for stays up to 90 days per period of
180 days the following:
- The two or multiple entry short-stay visa issued by a Schengen member state;
So if your Schengen Visa is a multi entry one, then you are ok to go.