"Voting for Trump is a morally good choice," he said as he declared his support for the Republican candidate ahead of November's election.
Trump faces Hillary Clinton in a tight battle for the White House.
Many Christians in America have said they cannot vote for Mr Trump because of his controversial views and questionable faith.
But others question Hillary Clinton's liberal - left leaning views on issues like gay marriage and abortion.
"I do not think that voting for Donald Trump is a morally evil choice because there is nothing morally wrong with voting for a flawed candidate if you think he will do more good for the nation than his opponent. In fact, it is the morally right thing to do," Mr Grudem writes in a blog post.
Trump, Grudem admits, "is egotistical, bombastic, and brash" but is a "good candidate with flaws".
He said: "He insults people. He can be vindictive when people attack him. He has been slow to disown and rebuke the wrongful words and actions of some angry fringe supporters.
"He has been married three times and claims to have been unfaithful in his marriages. These are certainly flaws, but I don't think they are disqualifying flaws in this election."
Donald Trump has faced intense pressure over comments he has made about immigrants, woman and gay people.
But this was a "hostile press exaggerating some careless statements he has made," Grudem claimed.
"I think most of the policies he supports are those that will do the most good for the nation.
"Therefore the question that Christians should ask is this: Can I in good conscience act in a way that helps a liberal like Hillary Clinton win the presidency?
"Some may feel it is easier just to stay away from this messy Trump-Clinton election, and perhaps not even vote. But the teachings of Scripture do not allow us to escape moral responsibility by saying that we decided to do nothing."
He said many Christians did not understand the ramifications of allowing Hillary Clinton into the White House.
Her changes to the country's Supreme Court would mean liberals would win legal challenges with more conservative Christians, he claims.
It would, he says, result in "judicial tyranny of the type we have seen when abortion rights and same-sex marriage were forced on the nation".