The application of biostimulant is a modern agronomic tool with several benefits for horticultural production. In the present study, we evaluated the effect of five biostimulant products with varied composition (e.g. seaweed extracts+macronutrients+amino acids (SW); humic+fulvic acids (HF); Si+Ca (SiC); Si (Si); vegetable proteins+amino acids (VP)) and control treatment (no biostimulant added (NB)) on field grown lettuce plants (Lactuca sativa L.: Romaine type cv. Doris) under deficit irrigation conditions (Control treatment: rain-fed plants; I1: 50% of field capacity; I2: 100% of field capacity). The experiment took place on the spring-summer growing period of 2021, at the experimental field of the University of Thessaly, Greece. Plants were transplanted on April 1, while harvest took place on May 27. The growth parameters tested were plant weight (aerial part), number of leaves, fresh and weight of leaves, leaf area index (LAI) and specific leaf area (SLA), and SPAD index. A varied effect of biostimulant treatment was observed on SPAD index and plant height. In particular, SPAD value increased when plants treated with vegetable proteins+amino acids (VP) at rain-fed conditions or seaweed extracts+macronutrients+amino acids (SW) at deficit irrigation (I1: 50% of field capacity). Plant height increased for the combinations of Si application at deficit irrigation or Si+Ca (SiC) at rain-fed conditions. Moreover, total plant weight, weight of leaves and LAI were the highest in the half irrigation treatment (I1), whereas the number of leaves increased for the rain-fed plants treated with humic+fulvic acids (HF) or for those that received half irrigation (I1) and Si. The highest dry matter content and SLA values were recorded for plants that did not receive biostimulants under rain-fed or full irrigation (I2). In conclusion, our results indicate a varied response to biostimulant application and irrigation conditions, where SW and Si treatments resulted in the highest overall yields under deficit irrigation. Moreover, the application of biostimulants alleviated the negative effects of deficit irrigation on leaves’ chlorophyll content which is associated with visual quality of the final product.