Water deficit can negatively affect physiological and biochemical responses in plants. The stress-mitigation potential of seaweed extracts (SWEs) Caulerpa lentillifera (green seaweed) and Gracilaria edulis (red seaweed) on oil palm seedlings subjected to water deficit was investigated. Plant height, relative water content, chlorophyll content, soluble protein concentration, and the expression of genes coding for thiamine (vitamin B1) biosynthesis enzymes were evaluated. SWE treatments were applied to four-month-old oil palm seedlings under full irrigation (control) and water deficit conditions. Overall, SWE did not affect the height of the seedlings after 60 days of treatment, but the seedlings managed to maintain relative water content of up to 90.00%, and chlorophyll and leaf protein content increased up to 50.00% in seedlings treated with red seaweed extract compared to the control. The application of G. edulis and C. lentillifera SWE induced downregulation of THIC (up to 15.00%) and THI4 (up to 15.46%) expression respectively on day 2 posttreatment as compared to the control. This finding is in line with the postulated role of thiamine in protection against stress in plants and suggests the role of SWE as a biostimulant.