Seymour, who has led the department for 15 years, had been on paid administrative leave since late August. He left the council meeting shortly after the vote and did not comment.
An independent investigator hired by the city said the investigation began with an anonymous letter, but many of the allegations were also detailed in Seymour's divorce records.
In those court records, Seymour admitted to having a sexual relationship with a captain in the Fulshear Police Department. The investigator said Seymour also spent money on the captain, including plane tickets, romantic trips to Galveston and Fredericksburg, flowers, and a refrigerator. The captain has since resigned.
During a presentation to City Council, the investigator said he sustained findings of untruthfulness and deception against Seymour. He said he also found that the chief violated policies involving financial conflicts with subordinates and failed to disclose his relationship with the captain.
The investigator also alleged Seymour discussed the investigation with department employees while it was underway despite being ordered not to do so.
Seymour has been chief since 2011 and oversaw the growth of the department as the Fort Bend County city rapidly expanded.
Councilmember Randy Connor addressed the decision after the vote.
"Trustworthiness is the cornerstone of government at every level, and when that trust placed in our public officials is violated, the credibility of government is diminished and, in some cases, lost altogether," Connor said.
Seymour had the option of having the council consider the matter in closed session but requested that the proceedings be held publicly.
His attorney, Larry McDougal Jr., said they disagreed with the council's decision and will review what legal options may be available.
Earlier Tuesday, the city told ABC13 that the city manager and Fulshear Police Department leadership were carrying out Seymour's duties.