Fine-grained, Ti-poor titanomagnetite in the ~12.7 Ma Tiva Canyon (TC) Tuff systematically increases in grain size from superparamagnetic (SP) at the flow base to single domain (SD) at a few meters height. This allows us to examine the role of grain-size variation on paleointensity, within the transition from SP to stable SD. We present magnetic properties from two previously unreported sections of the TC Tuff, as well as Thellier-type paleointensity estimates from the lowermost ~7.0 m of the flow. Magnetic hysteresis, frequency-dependent susceptibility, and thermomagnetic data show that sample grain-size distribution is dominated by SP in the lower ~3.6 m, transitioning upwards to mostly stable SD. Paleointensity results are closely tied to stratigraphic height and to magnetic properties linked to domain state. SD samples have consistent absolute paleointensity values of 28.5 ± 1.94 μT (VADM of 51.3 ZAm2) and behaved ideally during paleointensity experiments. The samples including a significant SP fraction have consistently higher paleointensities and less ideal behavior but would likely pass many traditional quality-control tests. We interpret the SD remanence to be a primary thermal remanent magnetization but discuss the possibility of a partial thermal-chemical remanent magnetization if microcrystal growth continued at T < Tc and/or the section is affected by post-emplacement vapor-phase alteration. The link between paleointensity and domain state is stronger than correlations with water content or other evidence of alteration and suggests that the presence of a significant SP population may adversely impact paleointensity results, even in the presence of a stable SD fraction.