This dataset contains individual-level growth rate measurements of the blue mussel Mytilus edulis and the Pacific oyster Magallana gigas, collected during a controlled mesocosm experiment in Sylt, Germany (coordinates are included in the dataset). Data were collected over a three-month period, from 29 March 2023 to 26 June 2023. Twelve mesocosms were used: six maintained at ambient temperature and six maintained at +3 °C above ambient. A randomized experimental design was used: six mesocosms contained only M. edulis, while the remaining six contained both species. Observations include shell length and daily shell-length growth rates, total wet weight (including shell) and daily growth rates based on weight. Each mussel was labeled and categorized by size class. Measurements were obtained using digital calipers (The Noble Finn, Model 150 mm) and laboratory balances (Type 00AC, Sartorius AG Göttingen, Germany), as well as handheld multiparameter instruments for water temperature. The dataset provides reproducible, traceable individual-level responses of M. edulis and M. gigas under controlled warming, supporting studies on physiological responses of coastal bivalves to temperature changes.