Background: Fear of COVID-19 has been associated with adverse mental and physical health outcomes, yet evidence from The Netherlands is limited. This study investigated associations between fear of COVID-19, mood, quality of life, immune fitness, and related health variables during the first Dutch national lockdown and identified key predictors of fear. Methods: In June–July 2020, n = 1020 Dutch adults completed an online survey assessing demographics, personality, mental resilience, pain sensitivity, pain catastrophizing, alcohol use, immune fitness, and mood. Retrospective ratings were provided for the pre-pandemic period (January–March 2020) and the first lockdown (March–May 2020). Fear of COVID-19 was measured using a modified Fear of COVID-19 Scale. Results: Overall, 13.2% of participants reported significant fear of COVID-19, which was associated with poorer mood, reduced quality of life, lower immune fitness, more severe COVID-19 symptoms, greater pain sensitivity, and higher levels of pain catastrophizing. Regression analysis explained 19.6% of the variance, with pre-pandemic anxiety (8.7%) and poorer immune fitness (3.4%) as the strongest predictors of fear of COVID-19, followed by lower psychoticism, lower mental resilience, older age, greater helplessness, and greater extraversion. Discussion: These findings suggest that a minority experienced high levels of fear of COVID-19 with substantial consequences, including negative effects on mood, immune fitness, and quality of life. The strong association with pre-existing anxiety and immune fitness highlights the need for early identification and targeted interventions for vulnerable groups to reduce psychological and physical health impacts in future public health crises.