WASHINGTON — The Department of Homeland Security will soon begin collecting social media data from all immigrants entering the United States, part of what agency officials call an effort to more effectively screen those coming to the country but privacy advocates see as an unnecessary intrusion that would do little to protect national security.

The department will begin collecting the information on Oct. 18, the same day the Trump administration’s new travel ban on citizens of seven countries and restrictions on those from two others are set to take effect.

Green card holders and naturalized citizens will also have their social media information collected, with the data becoming part of their immigration file. It was unclear whether the monitoring would take place only in the application process or could continue afterward.

The department published the new requirement in the Federal Register last week, saying it would collect “social media handles, aliases, associated identifiable information and search results,” which would be included in an applicant’s immigration file. It said the data would come from “publicly available information obtained from the internet, public records, public institutions, interviewees, commercial data providers.”

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The data collection has alarmed privacy groups and lawyers, who expressed concerns on about how the department would use the information. Advocates say they also worry that the monitoring could suck in information on American citizens who communicate over social media with immigrants.

“This would undoubtedly have a chilling effect on the free speech that’s expressed every day on social media,” Faiz Shakir, the national political director for the American Civil Liberties Union, said in a statement. “This collect-it-all approach is ineffective to protect national security and is one more example of the Trump administration’s anti-immigrant agenda.”

Efforts to collect social media information are not unique to the Trump administration. During the Obama administration, the department had begun asking visitors to voluntarily provide social media information, and had four pilot screening programs.

After the December 2015 mass shooting in San Bernardino, Calif., counterterrorism officials and lawmakers grew increasingly worried about the use of social media by terrorist groups like the Islamic State, also known as ISIS or ISIL.

Many members of Congress, mainly Republicans, began urging the Department of Homeland Security to use data gleaned from visa applicants’ or asylum seekers’ social media accounts as part of the immigration process.

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Congressional Democrats also supported the effort to collect social media data on visa applicants.

“We believe these checks, focused on possible connections to terrorist activity, should be incorporated into D.H.S.’s vetting process for visa determinations, and that this policy should be implemented as soon as possible,” Democratic lawmakers said in a letter at the time.

The attackers in San Bernardino, Tashfeen Malik and her husband, Syed Rizwan Farook, had exchanged private online messages discussing their commitment to jihad and martyrdom, law enforcement officials said. But they did not post any public messages about their plans on Facebook or other social media platforms.

One of the pilot projects, which began in December 2015, screened the social media accounts of applicants for so-called fiancé visas, the program under which Ms. Malik entered the United States.

United States Citizenship and Immigration Services, the part of the Department of Homeland Security that approves citizenship and green cards, has previously used social media as part of the screening process for Syrian refugees, but only when the person was flagged because of a hit in an intelligence database or when questions were raised during an interview with immigration officials.

Faiza Patel, co-director of the Brennan Center’s Liberty and National Security Program at New York University, said although it was true that the Obama administration collected social media information, the new monitoring put in place by the Trump administration represented an escalation.

“What is different here is that it appears that they are monitoring people who are already in the United States — green card holders, for example,” she said. “And there is a lack of transparency of how they are using this data, which does heighten the concerns for this group of people.”