<p>This systematic review synthesizes 76 studies on the internationalization of professional service firms (PSFs), recognizing their evolving boundaries from traditional sectors (e.g., law, accounting) to “quasi-PSF” sectors often classified as knowledge-intensive business services (e.g., information technology and software development). It develops an integrative framework that highlights the distinct antecedents, strategies, and outcomes of internationalization across these contexts. Unlike more capital-intensive firms, PSFs operate within high knowledge intensity, low capital intensity, and strong institutional embeddedness, leading to unique internationalization pathways. Our analysis further reveals that PSF strategies are shaped not only by institutional conditions but also by sub-sectoral logics and firm-level capabilities. The review critically evaluates the dominant theoretical lenses employed in existing research—external, internal, and relational—showing that each offers valuable insights but also exhibits important blind spots. We suggest that understanding the internationalization of PSFs benefits from service-sensitive, multi-level perspectives that consider professional norms, relational capital, and institutional dynamics. Our review shows that cross-border strategies in PSFs are shaped less by capital flows and more by intangibles, institutions, and relationships—factors often underemphasized in conventional models—and we outline a future research agenda that emphasizes integrative perspectives, sectoral diversity, and outcomes beyond financial performance.</p>