Abstract The present work aimed to explore the optimized conditions of hydrothermal co-liquefaction (co-HTL) of the green seaweed “Enteromorpha clathrata (EN)” and the lignocellulosic agricultural waste “rice husk (RH)”. Separate hydrothermal liquefaction (HTL) of EN and RH showed bio-oil yields of 26.0% and 45.6%, respectively. However, co-HTL under optimized conditions showed significant increase in the bio-oil yield by 71.7% over that of EN, and insignificant difference with that of RH. Nevertheless, the conversion ratio of co-HTL showed 10.6% significant increase over that of RH. GC-MS results showed that main compounds of EN and RH bio-oil lump into the C15–C20 and C5–C12 regions, mainly representing carbon range of diesel and gasoline, respectively. Short-chain (C5–C12) and long-chain (C14–C20) compounds in the bio-oil obtained by co-HTL represented 72% and 28%, respectively. In addition, the ratio of aromatic compounds in the bio-oil of RH was reduced by 9.3% as a result of co-HTL. In conclusion, results suggested 50% ethanol as a co-solvent, 300 °C and 45 min as optimum conditions for co-HTL of EN:RH (1:1 w/w). The present study demonstrated an efficient route for co-HTL of 3rd generation feedstocks with 2nd generation feedstocks which will have a significant impact on large-scale applications.