The Maunder Minimum (A.D. 1645–1715) is a useful period to investigate possible sun—climate linkages as sunspots became exceedingly rare and the characteristics of solar cycles were different from those of today. Here, we report annual variations in the oxygen isotopic composition (δ¹⁸O) of tree-ring cellulose in central Japan during the Maunder Minimum. We were able to explore possible sun—climate connections through high-temporal resolution solar activity (radiocarbon contents; Δ¹⁴C) and climate (δ¹⁸O) isotope records derived from annual tree rings. The tree-ring δ¹⁸O record in Japan shows distinct negative δ¹⁸O spikes (wetter rainy seasons) coinciding with rapid cooling in Greenland and with decreases in Northern Hemisphere mean temperature at around minima of decadal solar cycles. We have determined that the climate signals in all three records strongly correlate with changes in the polarity of solar dipole magnetic field, suggesting a causal link to galactic cosmic rays (GCRs). These findings are further supported by a comparison between the interannual patterns of tree-ring δ¹⁸O record and the GCR flux reconstructed by an ice-core ¹⁰Be record. Therefore, the variation of GCR flux associated with the multidecadal cycles of solar magnetic field seem to be causally related to the significant and widespread climate changes at least during the Maunder Minimum.