In March 1537, Yoshimoto created an alliance with the Takeda clan, whom with the Imagawa family had been in a long state of war until the Ujiteru's generation, by taking the daughter (Joein) of the Shugo in the Kai Province named Nobutora TAKEDA, as his lawful wife (Kosun Alliance [an alliance between the Takeda clan in Kai Province and the Imagawa clan in Suruga Province]).
天文6年（1537年）2月、氏輝期までの抗争状態にあった甲斐国の守護武田信虎の娘（定恵院）を正室に迎え、武田氏と同盟を結んだ（甲駿同盟）。