Backed by Arabic music, the website also carried an advert plugging the extremists' Facebook page and boasting of other hacks they had carried out.
The group claimed to have successfully infiltrated webpages belonging to Philippine Airlines and the El Salvador government.
But doubts were cast on the authenticity of those behind the hack after they misspelt the word Muslim on their homepage.
And their latest hacking efforts drew derision from many of those logging on to the TravelWest site in the hope of using the journey planner.
One person wrote: "Oh dear, can't tell the difference between a local bus company and travel site. Pathetic. What price education huh?"
Another joked: "Can you fix this little error on the TravelWest site? Nice music by the way."
Jake Bradley wrote: "What time is the number 8 to the town centre?"
The site is jointly run by several councils including Bristol, South Gloucestershire and North Somerset.
The West of England Partnership (WEP), which looks after the site, said it had alerted its IT department as soon as it was flagged up.
Julia Dean, communication officer for WEP, said: "Glad to report everything is now back up and working."
It is unclear at this stage whether the incident will be reported to police.
Cyberterrorism is becoming an increasing concern with extremists able to wreak havoc globally at the press of a button.
A recent cyber-attack on the Sony Corporation saw the release of highly personal data about celebrities, employees and their families.
The hack was later blamed on North Korea, which was said to be angry about the impending release of the film The Interview, a comedy about to plot to assassinate the country's leader Kim Jong-un.
A group known as the Syrian Electronic Army has also carried out a number of cyber-attacks on Western media organisations.