In this paper, we analyze the randomness of the RECTANGLE cipher. RECTANGLE is a lightweight block cipher with 64-bit block size and variants key lengths of 80 and 128 bits. Lightweight block cipher requires less computing power than a block cipher algorithm which makes it more efficient to be implemented in low-resource devices. Randomness is an important property of a cryptography algorithm to make sure the output has no message pattern. The randomness testing was performed using the NIST Statistical Test Suite. A total of nine data categories were applied to generate 1,000 input sequences for each algorithm. RECTANGLE-80 and RECTANGLE-128 passed 98.73% and 98.48% of the randomness tests. Our analysis shows that both RECTANGLE variants seem to be non-random based on the 0.1% significance level. The experimental results from this paper identified some weaknesses that can be addressed in future research. � 2020 ACM.