A papal conclave was held from 24 February to 31 March 1829 to elect a new pope to succeed Leo XII, who had died on 10 February. Of the 50 eligible cardinal electors, all but eight attended. On the final ballot, the conclave elected Cardinal Francesco Castiglioni, the prefect of the Congregation of the Index. After accepting his election, he took the name Pius VIII.

It took a long time for the conclave to elect a new pope due to conflict between secular governments concerning who should be elected. Cardinal Emmanuele De Gregorio was the proposed candidate of the pro-French faction and the zelanti (conservative cardinals), whilst Cardinal Bartolomeo Pacca was proposed by the more moderate cardinals, but he was not accepted by the French government of the Bourbon Restoration, headed by King Charles X and Prime Minister Jean Baptiste Gay. Pacca was also seen by many in the conclave as being too gentle to be an effective pope.

Description

The conclave did not move rapidly. The arrival of Giuseppe Albani caused the votes to center on Francesco Saverio Castiglioni. With the supporters of both De Gregorio and of Pacca unable to secure enough votes to elect their candidate to the papacy, Castiglioni came to be seen as a suitable compromise candidate. Castiglioni had been close to election in the 1823 conclave as the representative of the politicanti (moderate cardinals) and had all the qualifications to become pope, though he had the problem of being in very poor health, but was not elected at the last conclave when the zelanti cardinals realized that he was quite close to Cardinal Ercole Consalvi.{{cite web|title=The Triple Crown: An Account of the Papal Conclaves - Leo XII (De la Genga)|url=http://www.pickle-publishing.com/papers/triple-crown-leo-xii.htm|author=Valérie Pirie}}{{cite web|title=The Triple Crown: An Account of the Papal Conclaves - Pius VIII (Castiglione)|url=http://www.pickle-publishing.com/papers/triple-crown-pius-viii.htm|author=Valérie Pirie}} Consalvi however was already dead by the time of the 1829 conclave having died during the pontificate of Pope Leo XII.

On 31 March, Cardinal Castiglioni was elected pope. Given that Castiglioni had been called Pius VIII by Pius VII even before his death in 1823, and that Leo XII had said that Castiglioni "some day was to be Pius VIII" in the 1823 conclave, it was therefore a foregone conclusion that he would take that papal name upon becoming pope.

class="wikitable sortable" style="width:230px; float:center; margin:auto;" \|+Cardinal electors by region
scope="col"\|Region !scope="col"\|Number
style="text-align:center" scope="row"\|Italy \|style="text-align:center"\|41
style="text-align:center" scope="row" data-sort-value="Europe, Rest of"\|Rest of Europe \|style="text-align:center"\|9
style="text-align:center" scope="row" data-sort-value="America, North"\|North America \|style="text-align:center"\|0
style="text-align:center" scope="row" data-sort-value="America, South"\|South America \|style="text-align:center"\|0
style="text-align:center" scope="row"\|Asia \|style="text-align:center"\|0
style="text-align:center" scope="row"\|Oceania \|style="text-align:center"\|0
style="text-align:center" scope="row"\|Africa \|style="text-align:center"\|0
scope="row"\|Total !50

class="wikitable sortable" style="width:230px; float:center; margin:auto;"

|+Cardinal electors by region

scope="col"|Region

!scope="col"|Number

style="text-align:center" scope="row"|Italy

|style="text-align:center"|41

style="text-align:center" scope="row" data-sort-value="Europe, Rest of"|Rest of Europe

|style="text-align:center"|9

style="text-align:center" scope="row" data-sort-value="America, North"|North America

|style="text-align:center"|0

style="text-align:center" scope="row" data-sort-value="America, South"|South America

|style="text-align:center"|0