•A sensitive method to measure nine urinary metabolites of organophosphate ester flame retardants (OPFRs) is presented.•The novel method is based on solid phase extraction and ultra performance liquid chromatography coupled to tandem mass spectrometry (UPLC-MS/MS).•The method detection limits were between 0.08 and 0.25ng/mL for target metabolites; average recoveries of isotope-labeled standards spiked in synthetic urine were between 77.6% and 108%.•Urinary OPFR metabolites were determined at low ppb levels in 24 urine samples collected from pregnant Canadian women. Over the last few years, the use of organophosphate flame retardants (OPFRs) has been on the rise; however, there are knowledge gaps in both the human health effects of OPFRs and levels of human exposure. Currently, human biomonitoring data on the levels of OPFR metabolites in the Canadian population are still non-existent. Herein we describe a novel method to measure nine urinary OPFR metabolites using solid phase extraction and ultra performance liquid chromatography coupled to tandem mass spectrometry (UPLC-MS/MS). The method detection limits were between 0.08 and 0.25ng/mL for target metabolites. The newly developed and validated method was applied to the analysis of 24 urine samples collected in 2010–12 from pregnant Canadian women. The most frequently detected OPFR metabolite in urine of study participants (detection frequency: 97%) was diphenyl phosphate (DPHP), with concentrations ranging between <0.13–25.2ng/mL, followed (75%) by the sum of two metabolites (DoCP: Di-o-cresyl phosphate and DpCP: Di-p- cresyl phosphate) of tricresyl phosphate, with concentrations between <0.13–4.38ng/mL. With the exception of desbutyl-tris-(2-butoxy-ethyl) phosphate which was not detected in any of the samples, all other OPFR metabolites measured were found among study participants with variable detection frequency, suggesting pregnant Canadian women may be exposed to OPFRs.