Purpose: Malnutrition and sarcopenia require dietetic and physiotherapy interventions. In this study, we aimed to compare interprofessional identity of dietitians and physiotherapists, as well as attitudes towards, facilitators and barriers for, and occurrence of interprofessional treatment of malnutrition and sarcopenia by both professions. Methods: A cross-sectional online survey was distributed from December 4, 2021 until January 31, 2022 through an international online network platform for professionals (LinkedIn). Practitioners working as dietitian or physiotherapist in a healthcare setting were eligible for participation. Outcome measures concerned perceptions regarding shared problem domains, interprofessional treatment, attitudes towards interprofessional treatment, interprofessional identity, facilitators, and barriers. A Chi2-test, Mann–Whitney U-test, and Spearman’s Rho correlation were calculated. Results: Data from 53 physiotherapists and 48 dietitians were included. Malnutrition is considered a shared problem domain by both professions (U = 1248.000; p = 0.858). While sarcopenia is treated by both professions (U = 1260.000; p = 0.927), physiotherapists consider sarcopenia more often a shared problem domain compared to dietitians (U = 1003.000; p = 0.044). Attitudes towards interprofessional treatment were mostly positive (73%, n = 35 and 87%, n = 46 respectively). Interprofessional identity of dietitians was lower compared to physiotherapists (median = 4.0 versus median = 4.3 respectively; U = 875.000, p = 0.007). This was explained by lower interprofessional belonging (median = 4.0 versus median = 4.8 respectively; U = 771.000, p < 0.001) and lower interprofessional commitment (median = 4.0 versus median = 4.3 respectively; U = 942.500, p = 0.023). Interprofessional identity was correlated with efficient means of communication (r = 0.30, p = 0.003) and bureaucracy (r = −0.21, p = 0.034). Other barriers reported included available time, financial compensation, interprofessional knowledge, and obtaining extra care. Most reported facilitators concerned role clarity, clarity of expertise, and willingness of others to collaborate. Conclusion: Dietitians and physiotherapists have different interprofessional identities, but both are advocates of interprofessional treatment. Both professions mostly treat malnutrition and sarcopenia individually and have different perceptions regarding sarcopenia as shared problem domain. Facilitators were mainly related to clarity and commitment while barriers were mainly related to resources.