Watanabe Moritsuna served as a bushō from the Sengoku to early Edo periods.  He was a retainer of the Tokugawa clan.  He served as a hatamoto for the Edo bakufu and appointed (by the bakufu) chief retainer of the Owari domain.  He was the lord of Terabe Castle in Mikawa Province and counted among the Sixteen Divine Generals of the Tokugawa.

The Mikawa-Watanabe were hereditary retainers of the Matsudaira clan and descendants of Watanabe no Tsuna, a bushō from Saga-Genji of the middle Heian period.  Based on genealogical records, Moritsuna was a descendant of Watanabe no Tsutō, a grandson of Tsutsui no Hisashi, the second son of Watanabe no Tsuna from the middle Heian period.

Moritsuna originated from the village of Urabe in the Nukata District of Mikawa Province.  From an early age, Moritsuna served Matsudaira Ieyasu (later Tokugawa Ieyasu) who was of the same age.  Moritsuna was skillful in spear fighting.  In 1562, at the Battle of Hachiman in Mikawa, during a defeat to a retainer of the Imagawa named Itakura Shigesada, he fought valiantly in the rear guard and thereafter was called Yari no Hanzō on a par with Hattori Masanari who was called Oni no Hanzō.

Moritsuna, however, was also an ardent follower of the Ikkō sect affiliated with the Hongan Temple.  In 1563, after the outbreak of the Mikawa Ikkō-ikki, Moritsuna, along with Hachiya Sadatsugu and other retainers who were followers of the sect, betrayed Ieyasu and joined the uprising.  After Ieyasu suppressed the uprising, he was pardoned for his betrayal and permitted to return to service for the Tokugawa.  Thereafter, Moritsuna participated in a majority of the important battles waged by Ieyasu.  At the Battle of Anegawa, he was the first to slay the enemy, making numerous contributions in battle thereafter.  Moritsuna served with the vanguard forces as the head of the ashigaru, or lightly armed foot soldiers, deploying for the Battle of Mikata-ga-hara, the Battle of Nagashino, and the Battle of Komaki-Nagakute.  At the Battle of Nagashino, he killed Yamamoto Kansuke, the lineal heir of Yamamoto Kansuke (written with a different character, a general in the Takeda army).

In 1590, after the Tokugawa clan was moved from Mikawa to the Kantō, Moritsuna was granted 3,000 koku in the Hiki District of Musashi Province.  In 1600, Moritsuna was recognized for his years of meritorious service with an increase of 1,000 koku to his fief.  In total, he had a fief of 6,000 koku equivalent to the stipend of thirty mounted soldiers and served as the unit commander of 100 ashigaru.

In 1608, upon direct orders of Ieyasu, he became an appointed chief retainer of Tokugawa Yoshinao, the ninth son of Ieyasu assigned as lord of the Owari domain.  In addition to 4,000 koku in Musashi, Moritsuna was granted by the Owari domain 5,000 koku in Yazako in Owari and, from the bakufu, 5,000 koku in Terabe in Mikawa.  Altogether, Moritsuna held 14,000 koku and resided in Terabe Castle.  During the Siege of Ōsaka, Moritsuna deployed for the Winter Campaign in 1614 and the Summer Campaign in 1615 during which he served as a guardian for Yoshinao, the lord of the Owari domain, during his first experience in battle.  In 1616, after the death of Ieyasu, he served on a direct basis to Yoshinao in Owari.  In 1620, Moritsuna died at the age of seventy-nine.

For his achievements, Moritsuna was later honored as a member of the Sixteen Divine Generals of the Tokugawa under the command of Ieyasu.