Chiba Masatane served as the twenty-fourth head of the Shimōsa-Chiba clan.
In 1495, Masatane was born as the lineal heir of Chiba Katsutane, a daimyō and the lord of Motosakura Castle in the Inba District of Shimōsa Province.
On 11/15 of Eishō 2 (1505), at the age of ten, Masatane attended his coming-of-age ceremony at the Chiba-Myōken Shrine. This same year, his grandfather, Chiba Noritane, died of illness. Soon after the demise of Chiba Suketane (Masatane’s great-grandfather), Noritane transferred the headship of the clan to Katsutane. The ceremony was held based on an awareness by Katsutane that, in the future, the headship would be transferred to Masatane. Four years later, Katsutane retired and entered the priesthood, whereupon Masatane succeeded him, although, behind the scenes, Katsutane continued to hold the real authority.
Around this time, an internal conflict erupted in the family of the Koga kubō that was an ally of the Chiba clan whereupon Ashikaga Takamoto ousted his father, Ashikaga Masauji and younger brother, Ashikaga Yoshiaki, and assumed the role of the Koga kubō. While Masatane supported Takamoto, Mariyatsu Nobukiyo, the sengoku daimyō of neighboring Kazusa Province, ushered in Yoshiaki and there were also actions in the Chiba family to act in concert. In 1520, Yoshiaki’s army captured Oyumi Castle, the base of the Hara clan serving as the head of senior retainers of the Chiba. The next year, he adopted the title of Oyumi kubō. As a result, the authority of the Oyumi kubō extended to the location of Chiba Castle in Inohana. Therefore, Masatane had to hold the coming-of-age ceremony for his lineal heir, Chiba Toshitane, at his base in Sakura.
Masatane responded by aligning with the Hōjō clan of Sagami who were a rapidly rising power in the Kantō and attempted to oppose the Oyumi kubō. Takagi Taneyoshi, originating from an illegitimate branch of the Chiba and serving as a retainer of the Hara clan, became a close associate and wed Masatane’s younger sister. In 1527, after settlement between Hōjō Ujitsuna and Ashikaga Yoshiaki, the Chiba, the Mariyatsu, and the Satomi clans joined. During this period, it is surmised that Masatane was under the command of Yoshiaki.
In 1532, following the death of his father, Katsutane, Masatane acquired the real authority in the family and received the daughter of Hōjō Ujitsuna as the formal wife of Toshitane.
In the eleventh month of 1534, Masatane took advantage of an internal conflict in the Mariyatsu clan to separate from Ashikaga Yoshiaki but was then attacked by Yoshiaki in the fourth month of 1535 and surrendered.
In 1537, at a ceremony to mark the opening of Kogane Castle, the base of Takagi Taneyoshi, Masatane visited the castle and had a tour and tea ceremony with Taneyoshi at Kōnodai Castle.
After a deterioration of the relationship between Ashikaga Yoshiaki (the Oyumi kubō) and Hōjō Ujitsuna, in the twelfth month of 1537, Masatane colluded with Ashikaga Haruuji and abandoned Yoshiaki again. In 1538, when Yoshiaki attempted to subdue the Hōjō clan, Masatane, in a manner similar to obeying orders from Ashikaga Haruuji (the Koga kubō), participated in the First Battle of Kōnodai. Upon learning of the death in battle of Yoshiaki, he advanced to Oyumi Castle and, after an eighteen-year hiatus, the castle was returned to the control of the Chiba clan.
As an outcome of this battle, the authority of the Hōjō clan extended to the Bōsō Peninsula, gradually exerting a significant influence upon the Chiba clan. After the death of Masatane, the Chiba became de facto retainers of the Hōjō clan. His children included, in addition to Toshitane and Tanetomi, Usui Tanehisa who inherited the Usui clan, Hara Taneie adopted by Hara Taneyoshi, and Unakami Tanemori adopted by Unakami Yamashiro-no-kami.
At the Kairin Temple in the city of Sakura in Chiba Prefecture, there is included among a group of stone pagodas from the Middle Ages a pagoda with a memorial inscription for Masatane.