Within adult physical activity (PA) interventions, enhancing social support can foster behaviour change (initiation and maintenance), reduce social isolation, build connections with similar others, and foster a sense of belonging and connectedness. Fitness professionals and PA participants are critical support providers and/or facilitators within interventions (i.e., group PA; one-on-one PA with a fitness professional). Yet there is little guidance available regarding effective supportive behaviours and strategies for fitness professionals and participants. There is a need to identify and synthesize available evidence reporting on behaviours and strategies to support adults in PA interventions. Thus, an overview of reviews was conducted to synthesize this literature. Two independent reviewers screened citations and identified 19 studies for inclusion. Data were extracted and synthesized, and 11 categories related to supportive behaviours and strategies were identified: feeling welcomed (e.g., speaking to new participants), making PA fun (e.g., framing the exercise as enjoyable), supporting participants PA through instruction (e.g., individualized coaching on technique), informational support (e.g., sharing tips to remain active), tangible support (e.g., giving rides to class), autonomy support (e.g., providing choices in class), emotional support (e.g., showing concern for participants’ feelings), modelling PA (e.g., seeing similar others participate in PA), accountability/commitment (e.g., following up with participants who miss class), encouragement (e.g., vocalizing praise, reinforcement), and creating opportunities to make friends (e.g., allowing time to socialize in class). These findings collectively highlight numerous supportive behaviours and strategies which will be used to develop evidence-informed resources for fitness professionals working with various adult populations.