Multiferroic properties have been investigated for single crystals of the triangular-lattice antiferromagnet NiBr2 with long-wavelength (similar to 7 nm) cycloidal spin structure whose spin-spiral plane is parallel to (001). X-ray diffraction revealed a magnetoelastic lattice modulation with half the periodicity of the magnetic modulation below the cycloidal ordering temperature (T-IC similar to 23 K), indicating the elliptically distorted nature of the transverse helix. Field-reversible spontaneous polarization (P) appears in the [1 (1) over bar0] direction perpendicular to the spin rotation axis (parallel to[001]) below T-IC. P shows nontrivial dependence on the magnitude and direction of the poling magnetic field (H), suggesting the possible H selection of the propagation vector of the helix from the sixfold-degenerate directions, even for the in-plane cycloidal spin structure, through magnetic domain control in the higher-temperature spin-collinear antiferromagnetic phase.