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ABSTRACT Objective Blueline Tilefish Caulolatilus microps have a relatively brief history of exploitation in United States Atlantic waters. A stock assessment in 2017 indicated that age-structured models, although preferred over age-aggregated models, should not be used without reliable maturity estimates and age data for Blueline Tilefish. However, there have been only four immature Blueline Tilefish collected to date. Therefore, we collaborated with the charter boat sector of the recreational fishery to target small (<400 mm total length), potentially immature Blueline Tilefish to better estimate sexual maturity. In addition, we combined known data sets on age and reproduction from fish that were sampled from the southeastern U.S. Atlantic (SEUS) and U.S. mid-Atlantic (USMA) regions to determine the overall spawning season, sex ratios, and sexual maturity of Blueline Tilefish. Methods Gonad histology was used to microscopically identify sexes and reproductive phases to determine the spawning season and sex ratios. Generalized additive models were used to analyze gonad weight–whole weight and gonad weight–month relationships. Length and age at maturity were estimated using logit, probit, and Gompertz models. Maturity was defined in four ways, based on different histological thresholds of oocyte stages, to separate fish that are expected to spawn (mature; n = 1,532) from those that will not spawn (immature; n = 9). Results Sex ratios were female skewed in both regions and deviated significantly from 1:1 in the USMA. Gonad weight increased with whole weight in both regions. Female spawning peaks were identified as May–June and August–September for the SEUS and USMA, respectively, but USMA sample sizes remained low in most months. Despite targeted sampling, sample size remained low for immature fish (n = 9 total) as well as for fish from suspected nonspawning months. The Gompertz model for length at maturity and logit model for age at maturity in maturity scenario 4, whereby females in early development with cortical alveolar oocytes present were considered to be immature and females in vitellogenesis were scored as mature, were considered the best fit models for maturity ogives. These models estimated length and age at 50% maturity to be 337 mm total length and 3.1 years, respectively. Conclusions Through a collaborative analysis of multiple data sets, it was possible to determine the spawning season, sex ratios, and sexual maturity of Blueline Tilefish in the SEUS and the USMA. Targeted sampling remains the only known method to acquire immature fish needed for better sexual maturity estimates, so we suggest that collaborative relationships with industry be maintained.