Water deficit (WD) is known to reduce the final mass of tomato fruit. Despite the recorded positive relationship between final fruit mass and cell number, the latter being determined in the pre-anthesis period of ovary development, very few studies have investigated the effects of WD on ovary growth before anthesis. In this study, cell division and expansion in tomato ovaries (cv. H1311) were studied under two irrigation regimes: a control regime that maintained soil water content at field capacity and a WD regime with a 50% reduction in water supply compared to the control. The ovaries were sampled at 6 and 2 days before anthesis, at anthesis, at 2 and 5 days after anthesis, and at breaker stage. Effects of WD were already significant in the pre-anthesis period and the mesophyll layers contributed 96% of the final loss of fruit mass under WD. Overall, the results suggest that cell expansion was the predominant contributor to the loss of fruit mass, although cell number decreased at fruit scale. This was due to the relative contributions of the different layers to fruit cell number and mean cell volume, and to the specific patterns of development and response to WD occurring in each layer. This study provides important information on the regulation of fruit growth processes under WD and will help better predict the impact of it on fruit yield.