"This position is much too important for anyone to be less than their best," wrote Rosenthal, considered Texas's most powerful prosecutor.
Rosenthal did not immediately return a telephone call seeking comment on Friday.
Rosenthal has endured public outcry for his head, including from protesters at a street demonstration as well as his own Republican Party officials, since a federal judge released the e-mails as part of a lawsuit against the Harris County Sheriff's Department.
The bizarre contents of his computer's inbox included campaign-related e-mails, as well as an e-mail that compared former President Bill Clinton to racist stereotypes of black men and others with video attachments that contained bloopers from porn movies.
Romantic e-mails he had sent to his secretary, with whom he acknowledged having an affair in the 1980s, were also released, but by mistake.
Rosenthal cited the negative attention the e-mails have brought on his family as another reason for his resignation.
"I have been trying to restore my family as a unit, but the constant media pressure had made that restoration more difficult," he said.
Rosenthal's resignation came hours after Lloyd Kelley, the attorney whose civil rights lawsuit against the Harris County Sheriff's Department resulted in the release of the e-mails, had filed another suit Friday asking that the prosecutor be removed from office on grounds of misconduct, incompetence and drinking on the job.
"What else could he do?" said Kelley. "None of this was just made up. I didn't send those e-mails. Those are his acts."
In his resignation letter, Rosenthal praised the work of the prosecutors in his office, adding, "As the saying goes, 'If I were asked to lead a charge on hell, I'd want these people in my ranks."
But community and religious leaders in Houston said the racist and sexist e-mails found on Rosenthal's computer were an indication of deeper problems in the district attorney's office and that others in the office should resign as well. They also were angered that Rosenthal in his resignation letter didn't apologize for the racist and sexist e-mails.
"I know why he would want to lead a charge on hell (with his prosecutors) and want them in his ranks, because Rosenthal's behavior has demonstrated he is the devil himself," activist Quanell X said of Rosenthal, a conservative Christian who campaigned for his job in 2000 on the notion that the death penalty was not only Texas law, but God's law.
Rosenthal was forced off the March 4 GOP primary ballot by the scandal but steadfastly refused to resign, saying "stupidity" was not grounds for quitting.
But the pressure built as hundreds of demonstrators protested outside his office and as his own assistant district attorney, Kelley Siegler, now a candidate for district attorney, said publicly he should resign. Her husband, Sam, had sent Rosenthal several of the offensive e-mails.
Rosenthal also was under threat of a federal contempt of court citation for the deletion of 2,500 other e-mails demanded as part of the lawsuit against the Sheriff's Department.
The contempt issue is still pending before a federal judge and Kelley said Rosenthal's resignation won't affect it.
But an investigation by the Texas Attorney General's Office on whether Rosenthal used his county e-mail account for political campaigning, a possible violation of the law, ended with his resignation, said spokesman Jerry Strickland.
Harris County Judge Ed Emmett said he was grateful Rosenthal had chosen "to put the interests of Harris County above his own."
"With the recent distractions surrounding Mr. Rosenthal now removed, the Harris County District Attorney's Office can return to its mission of seeking impartial justice on behalf of all our residents," Emmett said in a statement.
Gov. Rick Perry will appoint Rosenthal's replacement, said Perry spokeswoman Krista Piferrer.
Rosenthal, 62, spent his entire career in the district attorney's office after attending night law school. The scandal brought out allegations of racism within the district attorney's office. Black jurors were struck because, some defense attorneys and former prosecutors said, they were seen as soft on crime; code names for blacks were bandied about in e-mails and black leaders believed that prosecutors under Rosenthal worked to punish blacks more harshly than whites.
Rosenthal was caught up in an unrelated but simultaneous scandal when he successfully asked a judge to dismiss a grand jury's indictment of a state Supreme Court justice and his wife on charges they torched their house because of financial problems. Grand jurors have taken the unusual step of suing to get a judge to allow them to reveal the evidence that they believe implicated Justice David Medina.
Medina and his wife have denied the charges.
