We report on the performance of a new type of helical undulator based on bulk single-grain GdBa₂Cu₃O_7-δ high-temperature superconductors. The 104-mm-long prototype with a period length of 16 mm and a magnetic aperture of 4 mm achieves an on-axis field above 2.5 T at a temperature of 10 K. This undulator technology has the potential to change the design parameters of future x-ray free-electron lasers, making them more compact and powerful.