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One-dimensional (1D) spin chain systems have received special attention to discover novel magnetic ground states and emergent phenomena, while magnetic studies on rare-earth (RE)-based 1D spin chain materials are still rare. Here, we report the synthesis, structure, and magnetic behaviors of a family of tetragonal tungsten-bronze (TTB) structure K2RENb5O15(RE = Ce, Pr, Nd, Sm, Gd–Ho) compounds, which consist of a 1D linear spin-chain structure built by RE3+ions along the c-axis and well spatially separated by nonmagnetic K/Nb–O polyhedrons with large interchain distances of ∼8.80–8.88 Å in the ab-plane. In this family of K2RENb5O15compounds, the nearest-neighboring interchain exchange interactions through the RE–O–K/Nb–O–RE routes are much smaller than the intrachain ones via the RE–O–RE pathways, and the low-temperature magnetic results reveal the absence of long-range magnetic order down to 1.8 K for all compounds. Among them, K2GdNb5O15, with spin-only magnetic moment S= 7/2, exhibits a long-range magnetic order with TN∼ 0.31 K and strong spin fluctuations at low temperatures due to its low-dimension characteristics. Moreover, a large magnetocaloric effect under low field change (ΔB) of ΔB= 0–2 T is realized at temperatures below 1 K for K2GdNb5O15, making it an ideal candidate for adiabatic magnetic refrigeration applications at sub-Kelvin temperatures. The K2RENb5O15become a rare family of insulating RE-based magnets for exploring the novel 1D spin chain physics beyond the 3dTM-based counterparts in terms of their combination of low dimension, strong spin–orbital coupling, and rich diversity of RE ions.