In his verses, Muhammad Ibn al-Dheeb al-Ajami praised the Arab Spring revolts that toppled four authoritarian leaders. He also criticized Qatar's former absolute monarch, Sheikh Hamad bin Khalifa al-Thani, and mocked "sheikhs playing on their PlayStations".
The appeal process has now been exhausted and his only chance of release is for the Persian Gulf's state's new Emir, Sheikh Tamim bin Hamad al-Thani, to pardon him.
"This is the final sentence for Muhammad, there are no more appeals he now has to serve 15 years in prison," said on Monday Mohammed Rashid al-Ajami, a cousin of the accused who lives in Kuwait.
He said the court took its final decision in less than three hours.
In February, al-Ajami's sentence was reduced from life imprisonment to 15 years. Defense lawyer Najib al-Naimi, who represents al-Ajami was not available for comment.
His cousin said there had been no communication with the new Emir.
"But the Emir knows of the case for sure and has the ability to pardon anyone of Qatar's sons," he said.
Qatar, a natural gas producer and home to a US military base, has escaped the unrest seen in other Arab countries. But freedom of expression is tightly controlled and Qatar has no organized political opposition.
BA/BA