I Don't Believe Him/Her/You
 

I’m trying to determine what to use when saying: “I don’t believe him / I don’t believe her / I don’t believe you (Usted).

Does “le” refer to him, her, and you (Usted) in the following sentence?

No “le” creo.

Is it correct to use “lo” for “him” in this sentence?
Is it correct to us “la” for “her” in this sentence?

Any and all input is appreciated.

The object in this case is a direct object. Ask yourself, "What/Whom do you believe?" If you can answer "him, her or you" (or their plurals), you are dealing with a direct object.
The direct object pronoun for 'him' is 'lo'. For 'her', it is 'la'. For 'you (formal-Usted)', first determine the gender and then use 'lo' or 'la', accordingly.

No lo creo. = I don't believe it/him/you (formal, male).
No la creo. = I don't believe her/you (formal, female).
No los creo. = I don't believe them/you (formal, plural, all male or mixed company).
No las creo. = I don't believe them/you (formal, plural, all female).


If you're in a place where leísmo is commonplace, or tolerated, it's OK to use 'le' (but only if the direct object is a singular male (or of masculine gender)). The plural form is 'les'.

Thanks for the detailed explanation, Rusty.

I questioned what to use because I see "le" used in writing all the time in similar sentences, so I was unsure if I could use "lo" and/or "la." Now I have no doubt.

That clears up the matter.

Great!

I will have to disagree about "leísmo" and "loísmo" here: When you say "don't believe her/him", we say "no le creas" (Indirect Object), because in Spanish we believe things, not people themselves and we are rather talking about the thing they say, which we assume is false.
But we say "no lo/la creas" (Direct Object) when it's the thing itself that mustn't be believed.
So when talking about a person, it's "loísmo", rather than "leísmo" what sounds strange. ;)

- Juan te está mintiendo; no le creas.
- Ana es una mentirosa; no le creas.
- Los niños dicen que tú les diste dulces; no les creo.
- Tus amigas están equivocadas; no les creas.
- Esta noticia es falsa; no la creas.
- Lo que te dijeron es mentira; no lo creas.
- Qué historias más ridículas me has contado; no las creo. (Or "no te creo", if you mean to say "I don't believe what you're saying").
- Qué comentarios tan absurdos; no los creo.

Glad you disagreed. I forced English grammar on the verb creer!
Sorry about the confusion.

To wrap our English-wired heads around the Spanish indirect object used in creer algo a alguien, we need to insert words to make it work.

- Juan te está mintiendo; no le creas.
- John is lying to you; don't believe (it-mandatory English direct object pronoun, not necessary in Spanish) (of) him.

- Los niños dicen que tú les diste dulces; no les creo.
- The children say you gave them some candy; I don't believe (it-mandatory English direct object pronoun) (of) them.
-or- I don't believe (them when they say) (it-mandatory English direct object: (that) you gave them some candy).

Thanks for the clarifications and further explanations, Angelica and Rusty. It's perfectly clear and I understand it.