Abstract The first potential exoplanetary biosignature detections are likely to be ambiguous due to the potential for false positives: abiotic planetary processes that produce observables similar to those anticipated from a global biosphere. Here we propose a class of methylated gases as corroborative “capstone” biosignatures. Capstone biosignatures are metabolic products that may be less immediately detectable, but have substantially lower false-positive potential, and can thus serve as confirmation for a primary biosignature such as O2. CH3Cl has previously been established as a biosignature candidate, and other halomethane gases such as CH3Br and CH3I have similar potential. These gases absorb in the mid-infrared at wavelengths that are likely to be captured while observing primary biosignatures such as O3 or CH4. We quantitatively explore CH3Br as a new capstone biosignature through photochemical and spectral modeling of Earthlike planets orbiting FGKM stellar hosts. We also reexamine the biosignature potential of CH3Cl over the same set of parameters using our updated model. We show that CH3Cl and CH3Br can build up to relatively high levels in M dwarf environments and analyze synthetic spectra of TRAPPIST-1e. Our results suggest that there is a coadditive spectral effect from multiple CH3X gases in an atmosphere, leading to an increased signal-to-noise and greater ability to detect a methylated gas feature. These capstone biosignatures are plausibly detectable in exoplanetary atmospheres, have low false-positive potential, and would provide strong evidence for life in conjunction with other well-established biosignature candidates.