Routine Assessment of Patient Index Data 3 (RAPID3) is a composite index, very useful for assessment of disease activity of various rheumatic diseases including RA. If RAPID3 can also reliably measure disease activity in axial spondyloarthritis (axSpA), it may prove to be a practical and effective quantitative assessment tool in busy practices. We studied the association of RAPID3 with Bath Ankylosing Spondylitis Disease Activity Index (BASDAI). Patients with Ankylosing Spondylitis (AS) seen from 2007 to 2012 were classified as having AS or non-radiographic axial spondyloarthritis (nr-axSpA) using modified New York criteria and Assessment of SpondyloArthritis International Society criteria, respectively. Patients with simultaneous BASDAI and RAPID3 scores were enrolled (N = 112; 105 with AS, seven with nr-axSpA). Multiple regression and nonparametric receiver operating characteristics were used. Baseline mean (SD) BASDAI and RAPID3 were 4.2 (2.5) and 3.8 (2.3), respectively. Multiple linear regressions modeled a quadratic relationship between BASDAI and RAPID3 for 321 observations in 112 patients with axSpA (1) cross-sectionally: BASDAI predicted by RAPID3 (β = 1.171; s.e. = 0.113, p < 0.001) and RAPID3(2) (β = -0.037; s.e. = 0.014, p = 0.011) with an adjusted R (2) of 0.676; and (2) longitudinally: BASDAI predicted by RAPID3 (β = 1.196; s.e. = 0.111, p < 0.001), RAPID3(2) (β = -0.042; s.e. = 0.014, p = 0.004), and visit number (β = -0.142; s.e. = 0.038, p < 0.001) with an adjusted R (2) of 0.689. RAPID3 (correctly classified) corresponded to BASDAI scores of 2, 4, and 6: 1.40 (85.8 %), 3.33 (81.9 %), and 5.43 (87.1 %), respectively. RAPID3 correlates well with BASDAI in monitoring axSpA patients (including AS) in cross-sectional and longitudinal follow-up. Since it also correlates with measures of disease activity of other rheumatic diseases including RA, RAPID3 could be an attractive measure for assessing and monitoring disease activity of several conditions seen in busy rheumatology practices.