Background Research has shown that sedentary behavior (SB) may increase dementia risk, but it remains unclear whether concentrated moderate to vigorous physical activity (MVPA) can compensate such negative effects. This study aimed to explore the association between different MVPA patterns combined with SB time and the risk of dementia. Methods This prospective study used data from the UK Biobank cohort, which provided accelerometer-based physical activity data for a full week from February 2013 to December 2015. Participants were categorized into "weekend warriors (WW)" group, engaged in more than 50% MVPA (>= 150 min/week) on 1 to 2 days; inactive group (total MVPA < 150 min/week); and regular group, who met the recommended MVPA (>= 150 min/week) but not WW. The participants were further divided into six groups based on SB duration (>= 8.52 h/day or < 8.52 h/day). A multivariable Cox model was used to assess the relationship between these patterns and the risk of dementia, adjusted by age, gender, ethnicity, Townsend deprivation index, education level, employment status, alcohol consumption, smoking, BMI, and baseline comorbidities (including cardiovascular disease, hypertension, and diabetes). Results We included 91,948 participants without dementia at baseline. During a median follow-up of 7.93 years, 736 participants developed all-cause dementia. When the MVPA threshold was set at 150 min per week, 16,149 participants (17.5%) were classified as WW with long SB, 19,055 (20.7%) as regular with long SB, and 21,909 (23.8%) as inactive with long SB. Compared to inactive and long SB time, the WW group showed a reduction in dementia risk (WW with long SB time: HR = 0.69, 95% CI: 0.54-0.87, P = 0.002; WW with short SB time: HR = 0.74, 95% CI: 0.56-0.97, P = 0.029). And regular group with shorter SB time was associated with a lower dementia risk (HR = 0.75, 95% CI: 0.59-0.96, P = 0.021), but not in the group with longer SB time. Conclusions The WW pattern may help mitigate the dementia risk associated with prolonged SB, suggesting that the quality and intensity of physical activity are also important factors.