Born in 1558 in Owari Province as the eldest son of Hachisuka Masakatsu (Koroku); his mother was Taishoin (Lady Matsu). He first served Oda Nobunaga, then from around 1575 joined his father in the service of Hashiba Hideyoshi, distinguishing himself as a member of the elite 'Yellow Cape Corps' at battles such as Nagashino. He continued to earn merit in Hideyoshi's campaigns against the Mori and in the conquest of Shikoku. After the 1585 conquest, Hideyoshi offered the whole of Awa Province to Masakatsu, but when his father declined, Iemasa received the domain instead, becoming a daimyo of some 180,000 koku. Entering Awa in June of that year, he first occupied the formidable mountain fortress of Ichinomiya Castle, but judging it unsuited for governing the whole province or deploying naval forces, he began construction of a new castle, Tokushima Castle, on the delta at the mouth of the Yoshino River, completing it and moving his base there the following year (1586). He also organized the 'Nine Castles of Awa' — a network of satellite fortresses staffed by clan members and senior retainers — to guard against the Iyayama uprising and invasions from neighboring domains, cementing control of his territory. At Sekigahara he had already retired in favor of his son Yoshishige, but his contribution to the Eastern victory secured the Hachisuka hold on Awa. A tea master with close ties to Sen no Rikyu, he was famously dubbed the 'Old Badger of Awa' by Date Masamune for his shrewdness. He died on February 2, 1639, at age 81.