The reductive coupling of carbon monoxide (CO) by metal hydrides is of fundamental interest and practical importance. Herein we report an unprecedented hydrodeoxygenative cyclotetramerization of CO by a trinuclear titanium polyhydride complex [(C5Me4SiMe3)Ti]3(μ3-H)(μ2-H)6 (1). The reaction of CO with 1 at -78 °C gave an ethen-1,2-diyl species [CH═CH]2- through the hydrodeoxygenative dimerization of two molecules of CO, which upon cycloaddition to another two molecules of CO afforded a cyclobuten-3,4-diyl-1,2-diolate unit [C4H2O2]4-. The hydrogenolysis of the [C4H2O2]4- species with H2 yielded a tetrahydrocyclobuten-1,2-diolate species [C4H4O2]2-, which on heating at 100 °C gave a cyclobuten-2-yl-1-olate product [C4H4O]2-. The acidolysis of the [C4H2O2]4- and [C4H4O]2- species with HCl afforded γ-butyrolactone and cyclobutanone, respectively.