The siege of Louisbourg was a pivotal operation of the Seven Years' War (known in the United States as the French and Indian War) in 1758 that ended the French colonial era in Atlantic Canada and led to the subsequent British campaign to capture Quebec in 1759 and the remainder of French North America the following year.

Weather conditions in the first week of June made any landing impossible and the British were only able to mount a bombardment of the improvised shore defences of Gabarus Bay from a frigate. However, conditions improved, and at daybreak on 8 June Amherst launched his assault using a flotilla of large boats, organized in seven divisions, each commanded by one of his brigadiers. French defences were initially successful and after heavy losses, Wolfe ordered a retreat. However, at the last minute, a boatload of light infantry in Wolfe's division (i.e., members of Rogers Rangers) found a rocky inlet protected from French fire and secured a beachhead. Wolfe redirected the rest of his division to follow. Outflanked, the French retreated rapidly back to their fortress.

Continuing heavy seas and the difficulty inherent to moving siege equipment over boggy terrain delayed the commencement of the formal siege. In the meantime, Wolfe was sent with 1,220 picked men around the harbour to seize Lighthouse Point, which dominated the harbour entrance. This he did on 12 June. After eleven days, on 19 June, the British artillery batteries were in position and the orders were given to open fire on the French. The British battery consisted of seventy cannons and mortars of all sizes. Within hours, the guns had destroyed walls and damaged several buildings. On 21 July a mortar round from a British gun on Lighthouse Point struck a 64-gun French ship of the line, Le Célèbre , and set it ablaze. A stiff breeze fanned the fire, and shortly after Le Célèbre caught fire, two other French ships, L'Entreprenant and Le Capricieux, had also caught fire. L'Entreprenant sank later in the day, depriving the French of the largest ship in the Louisbourg fleet.

The next major blow to French morale came on the evening of 23 July, at 10:00. A British "hot shot" set the King's Bastion on fire. The King's Bastion was the fortress headquarters and the largest building in North America in 1758. Its destruction eroded confidence and reduced morale in the French troops and their hopes to lift the British siege.

On 26 July the French surrendered. Having fought a spirited defence, the French expected to be accorded the honours of war, as they had given to the surrendering British at the Battle of Minorca. However, Amherst refused, tales of the atrocities supposedly committed by France's native allies at the surrender of Fort Oswego and Fort William Henry probably fresh in his mind. The defenders of Louisbourg were ordered to surrender all of their arms, equipment and flags. These actions outraged Drucour, but because the safety of the non-combatant inhabitants of Louisbourg depended upon him he reluctantly accepted the terms of surrender. The Cambis regiment refused to honour the terms of surrender, breaking its muskets and burning its regimental flags rather than hand them over to the British victors. Brigadier-General Whitmore was appointed the new Governor of Louisbourg, and remained there with four regiments.

Louisbourg had held out long enough to prevent an attack on Quebec in 1758. However the fall of the fortress led to the loss of French territory across Atlantic Canada. From Louisbourg, British forces spent the remainder of the year routing French forces and occupying French settlements in what is today New Brunswick, Prince Edward Island and Newfoundland. The second wave of the Acadian expulsion began. The British engaged in the St. John River Campaign, the Cape Sable Campaign, the Petitcodiac River Campaign, the Ile Saint-Jean Campaign, and the removal of Acadians in the Gulf of St. Lawrence Campaign (1758).

The loss of Louisbourg deprived New France of naval protection, opening the Saint Lawrence to attack. Louisbourg was used in 1759 as the staging point for General Wolfe's famous siege of Quebec ending French rule in North America. Following the surrender of Quebec, British forces and engineers set about methodically destroying the fortress with explosives, ensuring that it could not return to French possession a second time in any eventual peace treaty. By 1760, the entire fortress was reduced to mounds of rubble. In 1763 the Treaty of Paris saw France formally cede Canada, including Cape Breton Island, to the British. In 1768 the last of the British garrison departed along with most of the remaining civilian inhabitants.