About 41 million eligible citizens are called upon to cast ballots, as several parties also hold internal consultations to select their candidates for a presidential vote in May.
Sunday's elections are being held to renew 103 seats in the upper chamber, or Senate, of the bicameral Colombian Congress, and 183 seats in the lower chamber, or House of Representatives, for the 2026-30 term.
The first preliminary results are expected on Sunday evening.
President Gustavo Petro's left-wing governing coalition, Historic Pact, became the strongest left-wing party in Congress in the 2022 election, but failed to secure a majority.
Recent opinion polls show the coalition is once again in the lead.
The vote is taking place against the backdrop of a tense security situation, rising crime, economic challenges and deep political polarisation in the South American country.
In the parallel presidential campaign, conservative senator Miguel Uribe Turbay was shot at an event in the capital Bogota in June 2025 and later succumbed to his injuries.
Colombia traditionally holds parliamentary elections before presidential elections, providing an initial indication of which political camps will likely dominate in the upcoming campaign.
This year's vote is seen as a barometer of the strength of Petro's alliance shortly before the end of his term - and of the political tailwind his candidate will have in the race to succeed him.
The country's first left-wing head of state is not allowed to run again under the constitution.
Petro's government had announced ambitious reforms in the health, labour and pension systems, several of which faced resistance in Congress or stalled.
Petro's term in office has also been marked by ongoing conflicts with the opposition.
Although Historic Pact currently leads in the polls, it is unlikely to achieve a majority, with analysts expecting another highly fragmented parliament.
The right-wing conservative Democratic Centre party of former president Alvaro Uribe - the father of the senator killed in 2025 - could once again play a key role.
Traditional parties such as the liberals and conservatives also play an important role and have often acted as kingmakers.
Observers also see the parliamentary elections as an indication of how Colombia's future foreign policy might look.
Petro had taken a more independent course, which led to occasional tensions with the US, traditionally one of Colombia's closest partners, and disagreements with President Donald Trump on issues such as migration, drug policy and Venezuela.
The presidential election is scheduled for May 31.
If no candidate receives more than 50 per cent of the vote, a run-off election is expected to follow on June 21.
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