Feeling positively connected to other people is a basic human need. If this need is threatened by feeling lonely, people might become more susceptible to conspiracy theories to help make sense of their surroundings. Simultaneously, conspiracy beliefs could lead to loneliness because they can strain existing relationships. Using two pre-registered longitudinal studies, we investigated the reciprocal relationship between loneliness and conspiracy mentality (Study 1, N = 1604) and the more malleable specific conspiracy beliefs (Study 2, N = 1502) during the COVID-19 pandemic. Random-intercept cross-lagged panel models showed that people who are, on average, lonelier are also more likely to believe in conspiracy theories. However, the data provided no support for the notion that conspiracy beliefs and loneliness predicted each other over time. The research helps to understand the thus far mixed evidence on loneliness and conspiracy beliefs and adds important insights to the literature on conspiracy beliefs and need deprivation.