RATIONALE: The coronavirus disease 2019 (COVID-19) outbreak has dramatically altered people's social lives due to social restriction measures taken to control the coronavirus spread. Early on, increased loneliness has been publicly discussed as a harmful psychological side effect of these measures. Due to the serious adverse health consequences of loneliness, it is essential to take these concerns seriously and investigate them systematically to allow for evidence-based decision making. Thus far, however, high-resolution empirical evidence of such harmful side effects is rare. METHODS: The present preregistered large-scale daily diary study assessed daily loneliness in 4,844 German adults between March 16 and April 12, 2020. RESULTS AND CONCLUSION: Daily loneliness slightly increased during the first two weeks since implementing the pandemic-related measures and slightly decreased thereafter. With increasing age, daily loneliness increased more strongly over the four weeks. Moreover, daily loneliness increased more strongly for parents compared to people without children. Thus, despite some increases in loneliness in some individuals, there was no linear increase in loneliness in response to the first pandemic-related measures in the present sample.