Santagnello, SB, Martins, FM, de Oliveira Junior, GN, de Sousa, JdeFR, Nomelini, RS, Murta, EFC, and Orsatti, FL. Resistance training–induced gains in muscle strength and power mediate the improvement in walking speed in middle-aged women who are breast cancer survivors. J Strength Cond Res XX(X): 000–000, 2021—(a) Ascertain whether lower muscle mass, strength (1 repetition maximum [1RM]), and power (Pmax) in middle-aged women who are breast cancer survivors (BCS), when compared with women of a similar age never diagnosed with cancer (WNC), are related with lower walking speed (WS). (b) Ascertain whether changes in WS are associated with changes in muscle mass, 1RM, and (or) Pmax after resistance training (RT) in middle-aged BCS. A cross-section study was performed. Twenty WNC and 21 BCS were evaluated for lean mass of legs (LLM), 1RM (knee extension), muscle quality index (MQI = 1RM/LLM), Pmax (maximum muscle power–knee extension), and fast WS (10 and 400-meters). Randomized clinical trial was performed. The BCS were randomly divided into the control group (n = 9) and the RT group (n = 11). Breast cancer survivors exhibited lower 1RM (24.2%, p < 0.001), Pmax (30.6%, p < 0.001), MQI (22.2%, p = 0.001), and WS (10-m = 17.0%, p < 0.001 and 400-m = 10.5%, p = 0.002) than WNC. Resistance training increased 1RM (31.6%, p = 0.001), MP (29.0%, p = 0.012), MQI (28.5%, p = 0.008), and WS (10-m = 9.4%, p = 0.009 and 400-m = 6.2%, p = 0.006) in BCS. The changes in WS were positively associated with 1RM (10-m = 68%, p = 0.001 and 400-m = 37%, p = 0.036) and Pmax (10-m = 56%, p = 0.005 and 400-m = 40%, p = 0.027) and MQI (10-m = 63%, p = 0.043 and 400-m = 37%, p = 0.035). Resistance training–induced gains in muscle strength and power mediate the improvement in WS in middle-aged BCS. Resistance training is an effective strategy to improve WS in middle-aged BCS.