Toshimasa Niiro is a Japanese voice actor who has contributed to a number of anime series, with a notable concentration in the horror and anthology genres. His career in voice work is closely tied to the long-running supernatural horror series Theatre of Darkness Yamishibai, also known as Yamishibai Japanese Ghost Stories. In this series, which began airing in 2013 and continues to produce new seasons, Niiro has been a recurring and versatile cast member. He has voiced a variety of characters across multiple seasons, including taking on roles in the eighth, ninth, eleventh, twelfth, thirteenth, and fifteenth seasons. His characters in this series are often supporting figures in the short, standalone ghost stories, with credits including roles such as a grandfather, a father, a doctor, a spirit, and an elderly man.

Outside of his extensive work on Yamishibai, Niiro is also recognized for a role in the original television anime That Is the Bottleneck, which aired starting in 2020. In this series, he provides the voice for the character known as the Store Manager or Tenchou. This character is the heavyset, bespectacled manager of the convenience store Hot Hot Mart, and he is always depicted wearing a face mask. The show's cast was noted for including many veterans of stage productions, a background that applies to Niiro as well. His film voice acting credits include a role as a Retainer in the movie BanZai! or Kino kantoku banZai!.

A review of his broader career shows that he has also worked as a live-action actor, with credits appearing in Japanese television dramas and films from the early 2000s. This experience has contributed to his work in voice acting. Recurring collaborations are a strong feature of his professional profile, particularly his long-standing association with the creative team behind the Yamishibai franchise. He has repeatedly worked alongside director Akira Funada and a stable of voice actors including Kanji Tsuda, Yutaka Shimizu, and Yuta Hoshino across many seasons of the show. His notable achievements include being a dependable voice actor within a successful and durable horror anthology, providing voices for a wide spectrum of characters, and securing a memorable supporting role in the comedy series That Is the Bottleneck.