Water scarcity is a major factor restricting agricultural production and irrigation globally, with sustainable agricultural development calling for less irrigation water use and more production per unit of water applied. Improved understanding of plant physiological responses to water stress, and the effect of irrigation frequency on plant biomass production and quality, may help to optimize irrigation scheduling. Glasshouse-grown basil (Ocimum basilicum L.) received three different irrigation strategies: well watered, WW (daily irrigation with full crop evapotranspiration, as control), sustained deficit irrigation, SDI (daily irrigation with 75% full crop evapotranspiration) and infrequent drought and re-watering, DRW (applying the same volume of water as SDI but every 6 days). Leaf water potential (Ψleaf) and shoot xylem sap ABA concentration ([ABA]xyl) were correlated with decreased stomatal conductance (gs) under both deficit irrigation treatments. While the relationship between gs and Ψleaf depended on irrigation frequency, gs consistently declined as [ABA]xyl increased, in both intact plants (under both irrigation frequency treatments) and detached shoots fed synthetic ABA via the transpiration stream. Thus ABA played a dominant role in mediating stomatal closure in response to soil water deficit. Both SDI and DRW increased plant water use efficiency (WUE), and significantly increased the foliar phenolic composition (caffeic acid by 9% and 12%, and rosmarinic acid by 6% and 10%, respectively). Compared to WW plants, SDI increased biomass production (by 8% and 18% in leaf area and dry weight) but negatively affected quality (an undesirable peppery taste, with a rubbery texture during chewing). Although DRW decreased biomass production (by 12% for both leaf area and dry weight), quality was improved (traditional taste and flavor with a slight sweetness). To summarise, basil can be cultivated with less irrigation, but with different effects on either yield or quality according to irrigation frequency.