There are 13 words that start with 'poster'.
| poster | postern | 
| posters | posterns | 
| posterior | posterity | 
| posteriors | posteriorly | 
| posterities | posteriority | 
| posteruptive | posteriorities | 
| posterolateral | |
Note: These 13 words that start with 'poster' are found from the USA & Canada version of the tournament Scrabble word list.