Orexin 1 receptor (OX1R) is thought to be involved in various body functions, including arousal maintenance and emotional control, but the full details of its function remain unknown. OX1R imaging with positron emission tomography (PET) would be useful in elucidating the orexin system including OX1R, but no PET probes targeting OX1R have been reported. We, therefore, designed and synthesized tetrahydroisoquinoline (THIQ) derivatives as novel PET probes targeting OX1R, and evaluated their utility. In an in vitro competitive binding assay, THIQ-1 and THIQ-2 showed significantly higher binding to OX1R (IC50 = 30 and 31 nM, respectively) than OX2R (IC50 = 160 and 332 nM, respectively). These features were also observed in a cell binding assay using [18F]THIQ-1 and [18F]THIQ-2, demonstrating their OX1R-specific binding property in vitro. In a biodistribution study using normal mice, the brain uptake of [18F]THIQ-1 was higher than that of [18F]THIQ-2, but further improvement is required for in vivo imaging with PET. Taken together, [18F]THIQ-1 and [18F]THIQ-2 have the potential to become useful imaging probes for PET targeting the OX1R, but require additional structural changes to improve their brain uptake.