Although self-regulation (SR) has been regarded as an important factor of teachers' learning and professional development, its relationship with teachers' knowledge for teaching (Technological, Pedagogical, and Content Knowledge, TPACK) remains unclear. The purpose of this study was to explore the interrelationship between Taiwanese secondary in-service teachers' SR and TPACK in the national context (N = 386). Descriptive analysis, Pearson's bivariate correlation, and canonical correlational analysis were used to examine the interrelationship between the two sets of variables. Results showed that teachers scored highest on monitoring capability and controlling capability (MC/CC) but lowest on Information and Communications Technology (ICT)-related SR. Moreover, the participants scored greatest on Content Knowledge (CK) but lowest on Technology Knowledge (TK) and Technological Pedagogical Content Knowledge (TPCK). In-service teachers' MC/CC and Reflection Capability correlated higher with CK and pedagogical content knowledge (PCK) but lower with TK and TPCK. Conversely, ICT was associated more significantly with TK and TPCK but less significantly with CK and PCK. Implications of this study were provided along with suggestions.