The flag of Sarawak, officially known as Ibu Pertiwi ('Motherland'), is the official flag of Sarawak, a state in Malaysia. It is based on the flag of the Raj of Sarawak of the White Rajah, and includes the yellow of Southeast Asian royalty — a similar yellow and diagonal black are in the flag of Brunei, although Brunei's yellow is of a brighter shade.

James Brooke, the first White Rajah sailed from England in Dec of 1838, on The Royalist, which was a ship registered with the Royal Yacht Squadron. As such, it was permitted to fly a White Ensign and a Royal Yacht Squadron burgee. Various authors have suggested that James may have used these flags to represent himself and possibly even Sarawak before 1848, but no evidence has been cited to prove this flag was ever used to represent Sarawak itself.

The first flag designed and made for Sarawak was first hoisted on 21 September 1848. Brooke held a ceremony at his house on the grounds of the Astana. During this ceremony "A new flag, which the Rajah had brought from England, was then unfurled for the first time – displaying a black and red cross on a yellow field. This was to be henceforth the national flag of Sarawak."

This flag was flown for almost six months before James sought the British Foreign Office's approval. On 14 March 1849 James wrote a letter to the British foreign secretary Lord Palmerston. James wrote "...I beg to acquaint your Lordship, that on my return to Sarawak, feeling how desirable such a measure would be, I hoisted a flag, and recommended its adoption by all native prahus and other vessels belonging to this country. I subsequently waited to ascertain before reporting to your Lordship, how far the native community was inclined to adopt the use of the flag, which is a yellow field, with a cross per pale red and black, and I am happy to add, that they have eagerly embraced this distinguishing mark of Country, which they look upon as a security to these vessels..."

In June 1849 Lord Palmerston approved the use of the Sarawak flag and forwarded James' description and depiction of the flag to the British Admiralty.

The first known record of a ship flying the Sarawak flag left Sarawak on 12 December 1851. It was the barque "Fater Sarawak" captained by Cowan and arrived in Singapore on 19 December 1851.

There has been confusion over the history of the colours of the cross on the flag because other authors have reported both purple and blue instead of black as the other half of the cross opposing the red half. For example,

Due to these inconsistent descriptions of the colour of the cross, along with other circumstantial evidence Wilfrid John Chater found in the State archives, he developed a theory that the Sarawak flag flown on 21 September 1848 bore a blue & red cross. He first published this theory in the Sarawak Gazette 30 Nov 1964 in an article titled "Flags" He concluded that since he found a document using the phrase 'change blue bunting to black' dated 6 May 1870, that Charles must have changed the colour of the flag on that date. This faulty conclusion has been re-printed in numerous sources since 1964.