BACKGROUND: Having more cognitive activities may prevent dementia, but its evidence of modulating the functional brain network is limited. This randomised controlled trial (RCT) investigated the effect of increased cognitive activity participation on the default mode network (DMN) in older adults who had already been having regular cognitive activity participation and experiencing subjective cognitive decline (SCD). METHODS: Community-living Chinese individuals aged 55-75 years with regular practice of Chinese calligraphy and screened positive for SCD (but negative for mild cognitive impairment or dementia) were randomly allocated to either the intervention or control group. Over 6 months, the intervention group doubled their weekly calligraphy practice time, while the control group maintained their usual amount of practice. The primary outcome was functional connectivities (FCs) of DMN, with pre-specified regions of interest including medial prefrontal cortex (mPFC), inferior parietal lobe (IPL), hippocampal formation (HF), posterior cingulate cortex (PCC), and lateral temporal cortex (LTC). FC changes were compared using repeated measures multivariate analysis of variance (MANOVA). This study is registered at the Chinese Clinical Trial Registry, ChiCTR1900024433. FINDINGS: Between 15 January 2020 and 31 December 2021, 112 individuals consented and completed the baseline assessment. The participants, who had a mean age of 66.3 (SD 4.3) years, with 83 (74%) being women, had been practising calligraphy for an average duration of 9.7 years before enrolment and, in the preceding six months, for an average of 3.1 hours per week. 96 (86%) completed the post-intervention fMRI scan. Significant between-group differences were observed in the FCs between mPFC and right LTC (group difference = 0.25 [95% CI = 0.06-0.44], p = 0.009), mPFC and right IPL (0.23 [0.06-0.39]; p = 0.007), left HF and right LTC (0.28 [0.002-0.57]; p = 0.04), and left HF and right IPL (0.34 [0.09-0.60]; p = 0.009). INTERPRETATION: Our findings, which reveal positive neuromodulatory effects with increased calligraphy practice, highlight the importance of engaging more in cognitive activities in late life for better brain health. FUNDING: Research Grants Council, Hong Kong (grant number 24114519).