The growth of the fitness industry observed in the last decade has been accompanied by the emergence of an occupation as a social media fitness influencer. The most popular are able to accumulate millions of followers. The marketing potential of fitness influencers is a subject of interest, not only for the fitness industry but also for other sectors offering products related to health, wellness, or healthy nutrition. However, the activities of fitness influencers related to the promotion of physical activity and healthy lifestyle converge with the aims of those promoting public health. The main objective of this study was to make an assessment of the determinants of regular access to fitness influencers’ sites (FIS) and their relationship with the health behaviors of young adult women. It was based on the data originating from an online survey on a representative sample of Polish women aged 18–35 years. Chi2 test, univariate, and multiply logistic regression models were used to determine the relationships between FIS and the variables related to the respondents’ characteristics of and their health behaviors. FIS were accessed by 29.3% of respondents (n = 1030) at least once a week. It was found that those living in cities with a population ranging from 20,000 to 100,000 were more likely to access FIS than those living in rural areas. Similarly, greater access was made by those in a high-income household rather than those with the lowest income, by those with inadequate rather than those with problematic health literacy and by those with high rather than low e-health literacy. The use of FIS was significantly associated with the consumptions of fruit and vegetables (OR, 95%CI: 2.77, 2.01–3.82), physical activity (1.74, 1.27–2.38), breast self-examination (1.54, 1.11–2.13), and also with the greater use of e-cigarettes (1.63, 1.09–2.43) and increased consumption of alcohol (1.37, 1.01–1.88). In conclusion, as access to Internet resources run by fitness influencers was a prevailing predictor of young adult Polish women’s health behaviors, FIS may play a potentially important role in promoting a healthy lifestyle in this population. However, it should be remembered that there are complex patterns of associations with specific behaviors, e.g., the use of e-cigarettes or alcohol consumption.