The team keep the same driver line-up for the third consecutive year, with Brazil's Felipe Massa partnering Valtteri Bottas of Finland.
Team principal Sir Frank Williams says it will be a "challenge" to retain the third-place finish in the constructors' championship over the past two seasons.
"But we are determined to keep improving because only winning will ever be good enough," he added.
Williams are likely to be challenged by Red Bull, Renault, McLaren and Force India this season as well as last year's top two of Mercedes and Ferrari.
The FW38 sports the same red, white and blue stripes as the past two editions and chief technical operator Pat Symonds says the team have addressed the poor performance of last year's car at slower circuits.
"The FW37 was a pretty effective car and so we concentrated on understanding the areas where we could improve it without losing the attributes which made it effective," he said.
"It is no secret that the low speed performance of the FW37 didn't match its high speed performance, so a lot of time was spent looking into why this was and is subsequently making changes, which we hope will improve the situation."
The 2016 season starts with the Australian Grand Prix in Melbourne on 18-20 March.