<p>Depression and dementia commonly co-occur, yet little is known about depression trajectories across dementia stages. We aimed to map depression occurrence before, during, and after dementia diagnosis, and to identify factors associated with depression among individuals with dementia. This study included 10,051 participants from the Swedish Twin Registry. Participants with incident dementia (n = 2677) were matched with up to 3 controls (n = 7374) by birth year and sex. Depression and dementia diagnoses and their dates were ascertained based on medical records from the National Patient Registry. Conditional Poisson regression estimated incidence rate ratios for depression, while generalized estimating equations examined odds ratios for factors associated with depression. Compared with controls, depression risk among participants with dementia began to increase 6 years pre-diagnosis (incidence rate ratio [95% confidence interval] 2.32 [1.24–4.35]) and peaked during the year of dementia diagnosis (10.38 [7.33–14.69]). Depression risk remained elevated but gradually declined over the following 4 years (3.10 [1.67–5.77]). Female sex (odds ratio 2.21 [1.63–2.99]), smoking (1.58 [1.20–2.08]), heavy drinking (1.88 [1.10–3.21]), and stroke (1.94 [1.31–2.88]) were associated with higher odds of depression before dementia diagnosis, whereas being single (1.71 [1.10–2.37]) and having a history of cancer (1.35 [1.05–1.79]) were associated with post-diagnosis depression. Overall, these findings indicate that depression risk rises before, peaks at, and remains elevated after dementia diagnosis, with specific demographic (sex, marital status) and health-related factors (smoking, alcohol use, stroke, cancer) contributing to its occurrence among individuals with dementia.</p>