Aims: To develop a quantitative, real-time PCR assay to detect the nifH gene of Methanobrevibacter smithii. Methanobrevibacter smithii is a methanogenic archaea found in the intestinal tract of humans that may be a useful indicator of sewage pollution in water. Methods and Results: Quantification standards were prepared from Meth. smithii genomic DNA dilutions, and a standard curve was used to quantify the target gene and calculate estimated genome equivalency units. A competitive internal positive control was designed and incorporated into the assay to assess inhibition in environmental extracts. Testing the assay against a panel of 23 closely related methanogen species demonstrated specificity of the assay for Meth. smithii. A set of 36 blind water samples was then used as a field test of the assay. The internal control identified varying levels of inhibition in 29 of 36 (81%) samples, and the Meth. smithii target was detected in all water samples with known sewage input. Conclusions: The quantitative PCR assay developed in this study is a sensitive and rapid method for the detection of the Meth. smithii nifH gene that includes an internal control to assess inhibition. Further research is required both to better evaluate host specificity of this assay and the correlation with human health risks. Significance and Impact of the Study: This research is the first description of the development of a rapid and sensitive quantitative assay for a methanogenic archaeal indicator of sewage pollution.