In general your passport has to be valid minimum 6 month beyond the length of the trip when traveling in Jordan. The cost of a single entry visa is 20 JD per person (around 30 USD), it is valid for 28 days. Travelers arriving and departing Aqaba, either through the port, the airport or at the crossing from Israel or Saudi Arabia, are granted a free visa to Jordan. Most nationalities can obtain the Jordanian visa at the airport or the land borders, except the Allenby Bridge/King Hussein Bridge. The visa has to be paid in cash in Jordanian Dinar, money exchange facilities are existent. Some nationalities need the visa prior to arrival, see the below list. This list is meant solely as a useful tool and the data within it is subject to change without prior notice. It is recommended that you check with the Jordanian diplomatic mission in your country prior to travel.
UPDATE: Started and effective on 1st of April 2014, visa fee for entering Jordan is now 40 JOD per person ( around 60 USD). This applies to all individuals arriving at Queen Alia Airport or any ground border (except Allenby). In Aqaba and Arava non-restricted nationalities still obtain free visa on arrival.
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