The soluble expression of recombinant proteins in Escherichia coli is vital for protein applications in biotechnology and pharmaceuticals. However, the use of E. coli for efficient heterologous protein expression is hampered by several factors, such as poor expression and protein aggregation. Changing the culture or purification conditions may alleviate these issues, but methods based on gene fusion technology offer unique opportunities to improve the production and purification of soluble proteins. Here, we develop a novel fusion tag based on Spy, a newly identified molecular chaperone that functions in the periplasm of E. coli in an ATP-independent manner to prevent protein aggregation and assist in protein folding. We found that the tandem fusion of Spy stands among the well-described best fusion partners, such as MBP and SUMO, in increasing the soluble steady-state levels of six heterologous passenger proteins. Moreover, an easily aggregated passenger protein remained soluble after the removal of the Spy tag, implying that chaperone-dependent folding occurred when the passenger protein was fused to Spy. Our work expands the toolkit of fusion tags and allows them to aid in the production of unstable proteins with industrial or clinical values.,Copyright ? 2019 Elsevier B.V. All rights reserved.