(1) Background: individuals may benefit from being involved in physical and athletic activities in order to improve their body appearance and promote their physical and mental health. This study aimed to investigate body image, body mass index (BMI) characteristics, social physique anxiety, self-esteem and possible correlations between the above factors. (2) Methods: 245 adults engaged in training programs in gyms, as well as in track and field, football and basketball athletic activities completed (a) a sociodemographic questionnaire which recorded their BMI values and utilized the (b) Body-Esteem Scale for Adolescents and Adults, (c) the Social Physique Anxiety Scale and (d) the Rosenberg Self-Esteem Scale. (3) Results: Females and individuals with higher BMI reported statistically significant lower body-esteem and greater social physique anxiety levels compared to males and individuals with lower BMI, respectively (p< 0.05). A total of 25.3% of our participants were labeled as “overweight”, while 20.4% had been overweight in the past. Significant differences were reported between body-esteem and social physique anxiety levels (p< 0.001); age (p= 0.001); BMI value (p< 0.001) and never having a problem with body weight (p= 0.008). Additionally, individuals with lower body-esteem and greater social physique anxiety levels presented lower global self-esteem (p< 0.001). (4) Conclusions: individuals’ engagement in physical activity promotes physical as well as mental well-being, contributing to an improved quality of life, which may be the most important issue for health care professionals.