A novel series of 2-acyl-3-carboxyl-tetrahydroisoquinoline derivatives were synthesized and biologically evaluated. Among them, (S)-2-{(E)-3-furan-2-ylacryloyl}-7-[(2E,4E)-5-(2,4,6-trifluorophenyl)penta-2,4-dienyloxy]-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (compound 17u) was identified as a potent protein tyrosine phosphatase 1B (PTP1B) inhibitor without peroxisome proliferator-activated receptor (PPAR) γ activation: PTP1B inhibition IC₅₀=0.19 µM and PPARγ ΕC₅₀>10 µM. Compound 17u exhibited mixed-type inhibition for PTP1B, and this mode of inhibition was rationalized by computational ligand docking into the catalytic and allosteric sites of PTP1B. Compound 17u also showed high oral absorption at 10 mg/kg (per os (p.o.), C_max=4.67 µM) in rats, significantly reduced non-fasting plasma glucose and triglyceride levels with no side effects at 30 mg/kg/d (p.o.) for 4 weeks, and attenuated elevations in plasma glucose levels in the oral glucose tolerance test performed 24 h after its final administration in db/db mice. In conclusion, the substituted 2-acyl-3-carboxyl-tetrahydroisoquinoline is a novel scaffold of mixed-type PTP1B inhibitors without PPARγ activation, and compound 17u has potential as an efficacious and safe anti-diabetic drug as well as a useful tool for investigations on the physiological and pathophysiological effects of mixed-type PTP1B inhibition.