In 1554, Yoshimoto arranged a marriage between his legitimate son Ujizane and the daughter of Ujiyasu HOJO (Hayakawadono), tying his family to the Takeda clan as well as the Hojo clan, and formed the Ko So Sun Sangoku Domei (alliance between three provinces; Kai, Sagami and Suruga) (this negotiation is also called the 'Tripartite alliance of Yoshimoto IMAGAWA, Ujiyasu HOJO and Shingen TAKEDA').
天文23年（1554年）、嫡子・氏真に北条氏康の娘（早川殿）を縁組し、武田氏・北条氏と互いに婚姻関係を結んで甲相駿三国同盟を結成した（この会談は善徳寺の会盟とも呼ばれている）。