Recent advances in Thulium fiber laser technology have resulted in the availability of a compact, inexpensive, tunable, mid-infrared laser for potential use in laser surgery. The objective of this study was to tune the Thulium fiber laser wavelength and corresponding optical penetration depth to match the tissue thickness, and thus produce full-thickness, watertight tissue closure during microsurgical laser welding of urinary tissues. 1-cm-length incisions were made, ex vivo, in porcine ureters. Thulium fiber laser radiation with a wavelength of 1873 nm, power of 550-650 mW, and 750-μm-diameter spot was delivered to the tissue in continuous-wave mode through a 600-μm silica optical fiber. The fiber was scanned over the weld site once at a rate of 0.1 mm/s using a motion controller and linear stage controlled by a PC. Optical coherence tomography, histology, flow rates, and temperature measurements were used to optimize and evaluate laser welding success. Histologic analysis demonstrated full-thickness welding of the ureteral wall. Weld success rates ranged from 67% (8/12) at an incident laser power of 550 mW to 91% (10/11) at 650 mW. Peak flow rates greater than 200 ml/min were measured, however, mean flow rates were only about 50 ml/min. Average tissue temperatures increased with incident laser power from 59-89oC. The tunable Thulium fiber laser may be useful for surgical applications requiring variable control of thermal coagulation depth, such as microsurgical laser tissue welding.