Objective The common whelk Buccinum undatum is of great commercial and ecological importance, but little information is known regarding the gear selectivity of the fishery. Understanding gear selectivity is key for the long‐term sustainability of a stock and has applications in stock assessments and fishery management. Methods We investigated the effect of gear (pots) on the selectivity of catch within the common whelk commercial fishery through a mark–recapture study and comparative gear trials across Wales. The size composition of the population in the water was estimated by dividing the numbers at size brought onboard by the selectivity of the pots. The selectivity of experimental pots (commercial pots that were modified to catch a wider size range) was estimated by comparing the size composition of whelks caught in the experimental gear to the estimated size composition of the population. Result Peak selectivity of commercial pots occurred around the current minimum landing size (65 mm total shell length) for common whelks, with selectivity sloping off gradually for larger individuals and rapidly for undersized individuals. Across regions, less than 30% of the catch from baited lay‐down pots was discarded. Experimental pots were found to have a more variable but generally dome‐shaped selectivity curve similar to that of the commercial pots. Conclusion The fishery appears to be well designed, with gear successfully targeting the legal‐sized catch, which is promising for the conservation of the stock if discards have a high survival rate. Experimental gear is ideal for stock assessment purposes, and methods developed to estimate the selectivity of the experimental pots reduce the need for further intensive mark–recapture studies as fishing gear evolves over time.