- Born: 1839/10/11
- Died: 1899/12/12
- Other Names: 楽山 (Rakuzan), 九郎麿 (Kuroumaro)
- Japanese: 池田茂政 (Ikeda Mochimasa)
Ikeda Mochimasa was the ninth daimyô of Okayama han.
The ninth son of Tokugawa Nariaki, lord of Mito han, he was a younger brother to the final shogun, Tokugawa Yoshinobu. Adopted by Ikeda Yoshimasa in 1863, he succeeded Yoshimasa as head of the clan and lord of Okayama domain later that same year, and took important steps in appointing positions to lower-ranking samurai, and overseeing the domain's modern/Western-style military preparations. That year, Shogun Tokugawa Iemochi traveled to Kyoto to meet with Emperor Kômei, and Ikeda was granted one character from the shogun's name, changing his name from the childhood name of Kurômaro to "Mochimasa."
Mochimasa sided with the sonnô jôi movement in the Bakumatsu period, but found himself trapped between competing loyalties. Even as he petitioned the Emperor to postpone taking action to actually "expel the barbarians" (jôi), he opposed the shogunate attacks on Chôshû han, contributing to the conflict as a loyal shogunal vassal, but only nominally, without sending any actual troops. When the Imperial rebels declared that Shogun Tokugawa Yoshinobu, Mochimasa's elder brother, should be hunted down and killed, Mochimasa escaped having to take part in such action by claiming illness and abdicating his position to Ikeda Akimasa, lord of a branch domain.
Following the Meiji Restoration, he served as deputy head of the Imperial Prosecution and Investigation Office (Danjôdai). He also played a role in the revival of Noh, and is known to have been adept at waka under the poet pseudonym Rakuzan; Mochimasa also helped compile the Tokugawa reiten roku, a collection of Tokugawa shogunate ritual records, alongside Date Munenari and Matsudaira Shungaku.
Mochimasa died on December 12, 1899 at the age of 61.