What is the Japanese pickled plum often used in rice balls?

The Japanese pickled plum often used in rice balls is umeboshi.

Umeboshi are salted and pickled ume fruits, traditionally dried after brining. Although ume is commonly translated as “Japanese plum,” Prunus mume is botanically closer to an apricot. The finished fruit is intensely sour and salty, though modern versions may be sweetened with honey or reduced in salt.

Umeboshi are a classic filling for onigiri, where the sharp flavor contrasts with mild rice. A single red umeboshi placed in a bed of white rice creates a design resembling Japan’s flag; this presentation is called a hinomaru bentō. Umeboshi may also be served alongside rice or used in sushi and other dishes.

They are sometimes confused with tsukemono, nukazuke, or takuan. Tsukemono is the broad Japanese category of pickled foods, while nukazuke describes bran-pickled foods and takuan is pickled daikon radish. Umeboshi is the specific name for pickled ume.