Ōkubo Tadamasa (大久保 忠方, June 15, 1692 – November 20, 1732) was the 4th daimyō of Odawara Domain in Sagami Province, (modern-day Kanagawa Prefecture) in mid-Edo period Japan.
Tadamasa faced the daunting task of attempting to reduce the massive debt incurred by his father to the Tokugawa shogunate due to the Great Genroku earthquake and the Hōei eruption of Mount Fuji, and associated aftershocks, crop failures and floods.
Although he encouraged the migration of artisans to Odawara and the opening of new rice lands, high taxation and increasingly severe inflation led to civil unrest in Odawara-juku.
Tadamasa died of illness on November 20, 1732, at the domain's Edo residence, His grave is at the clan temple of Saisho-ji in Setagaya, Tokyo.
Tadamasa was married to an adopted daughter of Yanagisawa Yoshiyasu, the senior advisor to Shōgun Tokugawa Tsunayoshi.