was a Japanese noble and scholar of the late Nara period. He was the grandson of the sadaijin Isonokami no Maro and son of the chà «nagon Isonokami no Otomaro. Yakatsugu himself reached the court rank of and the position of dainagon, and was posthumously awarded the rank of .
In 751, Yakatsugu was promoted from to and appointed . In 757, he was promoted to and appointed governor of Sagami province, in 759 the governor of Mikawa province, and in 761 the governor of Kazusa province. In 761 he was also appointed vice-envoy to Tang dynasty China, but in the next year was replaced by Fujiwara no Tamaro without making the voyage over. In 763, he was appointed vice-minister in the Shikibu-shà Â.
Yakatsugu plotted with Fujiwara no Yoshitsugu, à Âtomo no Yakamochi, and Saeki no Imaemishi to overthrow the daijà Â-daijin and power of the day, Fujiwara no Nakamaro, but their plot was discovered. In 764, Yakatsugu was demoted to an assistant minister in the Dazaifu â until nine months later, when Nakamaro led his own failed rebellion. Yakatsugu was rehabilitated, promoted to , and made governor of Hitachi province.
Under Dà Âkyà Â's ensuing government, Yakatsugu was steadily promoted, rising in 765 to and , and joining the ranks of the kugyà  with a promotion to sangi in the next year, 766. Later that year, he was promoted to , and in 768 to .
Upon the death of Empress Shà Âtoku in 770, Yakatsugu aligned with Fujiwara no Nagate in supporting the future Emperor Kà Ânin as her successor. The appreciative new emperor continued to value him, and Yakatsugu was promoted to chà «nagon in 771 and dainagon in 780, making him the third highest ranking figure in the daijà Â-kan after udaijin à Ânakatomi no Kiyomaro and naidaijin Fujiwara no Uona. During this period, he also held positions as head of the Dazaifu, shikibu-sho, Ministry of the Center, and Crown Prince's Quarters. In 775, he reclaimed his family's old name of Mononobe no Ason, but in 779 he changed it back to Isonokami, now as .
In 781 Yakatsugu was promoted to , but died two months later. He was posthumously awarded the rank of .
Yakatsugu also exhibited a fine appearance. His speech and behavior were calm and elegant.
A great lover of the Confucian classics and historical texts, he was familiar with a wide variety of books. He also liked to write, and was proficient in both cursive script and clerical script. He wrote kanshi, and was considered a leading scholar of his day along with à Âmi no Mifune. His works are included in the Keikokushà «.
He was also versed in Buddhism, and wrote and on the subject. He built a temple called on his former residence, and in a corner of the complex he established a library called . Here he made various texts available to the public, focusing primarily on non-Buddhist works. This is believed to have been Japan's first public library.