The word "Ajman" literally translates to the "Persians". There are no doubts regarding the tribe's Persian origins, however, whether or not they descend from a single forefather or that they are from different ancestries is disputed amongst scholars. Some have alleged that they are descended from a poor Persian man from Khurasan who migrated to northeastern Arabia, some link them to the remnants of the imperial Persian soldiers in Yemen who tried retaking that country from the Ethiopian empire, and others believe that they are the descendants of the Persian prisoners of war that emperor Yazegerd III placed in Al-Ahsa. The Solbis (Arabic: صلبي, i.e, people of unknown origins) are said to have joined this tribal confederation as well, most of whom which belong to the Al-Sulaiman clan.
The Ajman are known to have been a key ally of Yam and Al-Murrah (a sub-tribe of Yam).
The 'Ajman were noted for their strength in battle and were important players in the wars and politics of eastern and central Arabia in the 19th and early 20th centuries. Their most famous leader (or shaikh) during the 19th century was Rakan ibn Hithlayn, who is still well known in Arabian tribal lore, and who was noted for his poetry as well as aptitude in battle against the Ottoman Turks. The 'Ajman, however, were defeated by Faisal ibn Turki, the second Imam of the Second Saudi State, who later married into the tribe. Later on, they supported the cause of the Saud al-Kabir branch of the Al Saud against their cousin Abdul Aziz ibn Saud, the founder of Saudi Arabia.
A section of the Ajman led by Dhaydan ibn Hithlayn joined the Ikhwan movement in 1912, providing military support for Ibn Saud, but later rebelled against him. The Ajman and their allies from the tribes of Utaybah and Mutayr were defeated by Ibn Saud in 1929 in the Battle of Sabilla, which put an end to the Ikhwan rebellion.
Nearly all the Ajman have abandoned nomadic life and have settled in the Persian Gulf states, particularly the eponymous Emirate of Ajman—a member emirate in the United Arab Emirates. There are also many in Saudi Arabia. Their main tribal territory is Joudah, also known as Wadi el-Ajman ("the valley of the Ajman"), located on the road between Riyadh and Dammam.
Ajman attacked Sobyie tribe in 1764 who had to call Ibn Saud to protect them from Ajman tribe. Ibn Saud responded immediately and killed 50 and captured 240 persons of Ajman. Shikh Rakan Bin Hathleen sent two of his sons to Banu Yam in Najran asking them for help. Najran was such nightmare for Ibn Saud that time. Shikh Hassan Bin Hebat Allah was the religious leader of Yam and he responded to Ajman and called for the general mobilization to AD'deryah in Ryadh with 500 men on 500 black horses (one of their techniques in war). Yam reputation was terrifying every single tribe at that time which pushed Qahtan tribe to build an alliance with them. Yam arrived in Ryadh and joined Ajman and moved to Adderyah. Ibn Saud had 3700 men but Shikh Muhammed bin Abdulwahab warned Mohammed bin Saud and ask him to make peace with Yam but he fought them and was defeated. Yam killed about 390 men and captured 220 men and got the Ajman prisoners back from Ibn Saud. Ibn Saud had to make peace with Yam so that Yam will go back to Najran and Ajman will stay in Najd under Yam's full protection. Ibn Saud knows that Yam keep their word and fight for it so that he was not worry about them when they made peace. This battle named Al-Ha'ir (الحائر).
- .قال الملك عمر بن يوسف بن رسول ( ت 696 هـ ) في كتابه" طرفة الأصحاب في معرفة الأنساب "( ص 142 ) :" والعُجْمَان نسبتهم إلى العجم ، يقال أن جدهم فقير عجمي نجع من خرسان " .
- ذكر أمين الريحاني ( ت 1359 هـ ) في كتابه " تاريخ نجد الحديث " ( ص 18 ، 223 ) أن في بلاد فارس من يقول أن العُجْمَان منهم .
- .ذكر سالم الطويرقي في مقاله " أصل قبيلة العجمان " ( المجلة العربية 10/87 ) قولاً أن العجمان بقايا الجيش الفارسي الذي أمده به كسرى – ملك الفرس – سيف بن ذي يزن لاسترجاع حكم اليمن من يد الأحباش .
- ذكر دكسون في كتابه " عرب الصحراء " ( ص 365 ) قولاً أن العُجْمَان من ذرية سجناء الحرب من العجم الذين وضعهم كسرى في الأحساء .
- انساب العجمان