Spearman last month scored 341 for Gloucestershire against Middlesex - breaking England great WG Grace's record individual score of 318 not out for the western County that had stood since 1876.
Despite Spearman's latest hundred, Friday's match ended in a draw with Gloucestershire declaring their second innings with the score at 401 for seven , a lead of 494 runs. Spearman was lbw to off-spinner Paul Weekes.
Spearman, 32, played 19 Tests and 51 One-day Internationals for New Zealand. He called time on his international career three years ago and came to England to take up a banking job.
However, in 2002, Gloucestershire coach John Bracewell - now in charge of New Zealand - persuaded Spearman to resume his senior cricket career.
And next year Spearman, already allowed to play on the County circuit as a 'home' player thanks to his Welsh mother, will become qualified for England at international level, having served a four-year exemption from top-flight cricket.