Iwamatsu Masazumi served as a bushō during the Sengoku period.  Masazumi was a member of the Nitta-Iwamatsu clan and served as the lord of Nitta-Kanayama Castle in Kōzuke Province.

In 1485, Masazumi was born as the son of Iwamatsu Hisazumi, a daimyō and renga practitioner from the late Muromachi period and lord of Nitta-Kanayama Castle.

His father, Hisazumi, came into conflict with a retainer named Yokose Kageshige and, as a result, was forced into retirement.  Through the intervention of Ashikaga Shigeuji, the Koga kubō, and with the backing of the Yokose, Masazumi replaced Hisazumi as the head of the Nitta-Iwamatsu clan.  Owing, however, to his youth, as well as the circumstances of his ascension to the role, he became a puppet of the Yokose clan.

Consequently, in a bid to reclaim power, Masazumi clashed with Yokose Yasushige.  In 1529, a plot to assassinate Yasushige was discovered, whereupon Masazumi was subject to a counterattack by Yasushige, resulting in his death.  He was forty-five years old.

After the demise of Masazumi, Yasushige reconciled with his son, Iwamatsu Ujizumi, and supported him.  Ujizumi became a sengoku daimyō of Kōzuke Province.  Under an alternate theory, Ujizumi was Masazumi’s younger brother.