The structure of double gain arms is the typical arrangement for Mamyshev oscillators (MOs), which is complex and costly compared with common mode-locking cavities. To pursue a simpler structure, single gain-arm MOs attract researchers. Realizing a single gain-arm structure in erbium (Er)-doped MOs encounters difficulties due to the intrinsic dispersion properties of silica fiber at 1.55 µm. Here, by utilizing a broadband band-pass filter in the cavity structure, we numerically and experimentally demonstrate an all-fiber Er-doped MO with a single gain arm. This single gain-arm Er-doped MO has a low mode-locking threshold and delivers a maximum output power of ∼16.68 mW corresponding to a pulse energy of ∼2.27 nJ. The mode-locking pulses with a repetition rate of ∼7.36 MHz have a pulse duration of ∼113 fs. We believe our work opens a way to achieve a single gain-arm Er-doped MO, which is attractive for practical applications.