Abstract. For an integer k ≥ 2, let (F (k) n )n be the k−Fibonacci sequence which starts with 0, . . . , 0, 1 (k terms) and each term afterwards is the sum of the k preceding terms. In this paper, we find all k−Fibonacci numbers which are Mersenne numbers, i.e., k−Fibonacci numbers that are equal to 1 less than a power of 2. As a consequence, for each fixed k, we prove that there is at most one Mersenne prime in (F (k) n )n.