Motivational strategies are among the most promising approaches to improve the effectiveness of batterer intervention programs (BIPs). An individualized motivational plan (IMP) is one of these motivational strategies. The present study aimed to explore whether adding an IMP to a standard BIP improved the participant-facilitator working alliance and participants’ protherapeutic behaviors. To this end a randomized controlled trial was conducted. One hundred fifty-three men convicted of intimate partner violence were randomly assigned to either a standard BIP (control condition, n = 79) or a standard BIP plus IMP (experimental condition, n = 74). Working alliance (i.e., general working alliance, agreement, and bond) was assessed with the Working Alliance Inventory-Observer, short version. Protherapeutic behaviors (i.e., assumption of responsibility, participant role behavior, and group value) were assessed with the Observational Coding of Protherapeutic Group Behavior. Both working alliance and protherapeutic behaviors were assessed by an external observer early and late in intervention. Our results showed that both general working alliance and agreement and bond, were significantly higher in the standard BIP plus IMP intervention condition, both early and late in intervention. All protherapeutic behaviors were significantly higher in the standard BIP plus IMP early in intervention, and also late in intervention for assumption of responsibility and group value. Our findings have important practical implications as our results clearly showed that a motivational strategy tool such as the IMP improves key intervention processes (i.e., working alliance and protherapeutic behaviors) in BIPs, therefore increasing their effectiveness.