A method of liquid–liquid microextraction is proposed for the extraction and preconcentration of tetracycline antibiotics from milk. It involves the extraction of analytes into a hydrophobic deep eutectic solvent (DES) based on a terpenoid and a carboxylic acid, followed by back extraction into the aqueous phase. A synergistic effect was revealed in the microextraction of tetracyclines into a DES phase based on thymol and octanoic acid (molar ratio of precursors 0.35 : 0.65), i.e., higher distribution ratios compared to those calculated using the additive scheme. Possibilities of the proposed method are shown on an example of the determination of tetracycline, chlortetracycline, and doxycycline in milk samples by HPLC–MS/MS. The limits of detection (3σ) for tetracycline and doxycycline are 5 ng/kg and for chlortetracycline,—50 ng/kg.