Polysomnography (PSG) is the gold standard for sleep bruxism (SB) assessment, it is expensive, not widely accessible, and time-consuming. Given the increasing prevalence of SB, there is a growing need for an alternative, readily available, reliable and cost-effective diagnostic method. This study aimed to evaluate the diagnostic validity of portable respiratory polygraphy (PRPG) compared with PSG for SB diagnosis. One hundred and three subjects underwent simultaneous examinations using PRPG (NOX T3, NOX Medical) and PSG (NOX A1, NOX Medical) in a sleep laboratory. The mean Bruxism Episodes Index (BEI) measured by PRPG was 4.70 ± 3.98, whereas PSG yielded a mean BEI of 3.79 ± 3.08. The sensitivity for detecting sleep bruxism (BEI >2) by PRPG was 48.3%, with a specificity of 81.2%. The positive predictive value was estimated at 51.9%, and the negative predictive value at 78.9%. However, when distinguishing between mild bruxism (BEI >2 < 4) and severe bruxism (BEI >4), PRPG demonstrated a sensitivity of 77.8% and 68.3% and a specificity of 48.6% and 71.4%, respectively. Polysomnography continues to be the SB diagnostic gold standard tool, as the sensitivity and specificity of PRPG are significantly lower when compared with PSG. Nevertheless, PRPG could serve as an alternative tool for SB screening or diagnosis, despite its limitations. Furthermore, our data indicate that comorbidities such as sleep apnea and sleep quality do not influence the diagnostic accuracy of PSG, suggesting its potential as a screening instrument in individuals with other sleep disorders.