Proprietário vs. senhorio
While both words can often be translated as "landlord" in English, there is a fundamental functional difference. Proprietário refers to the legal status of ownership regarding any asset, whereas senhorio is a specific term describing the relationship the owner has with a tenant within a rental agreement.
Proprietário
B1This word literally translates to "owner" or "proprietor." It focuses on the legal title and the fact that the person owns the asset (whether it is an apartment, a car, or land). You use proprietário when the focus is on rights, legal documents, or ownership status, regardless of whether there is a tenant living there.
O proprietário decidiu colocar o apartamento à venda.
(The owner decided to put the apartment up for sale.)
Precisamos da assinatura do proprietário na escritura.
(We need the owner's signature on the deed.)
A reunião de condomínio é exclusiva para cada proprietário do prédio.
(The condo meeting is exclusive to each owner in the building.)
Ela é a proprietária legal deste terreno.
(She is the legal owner of this land.)
O proprietário não permitiu obras estruturais na casa.
(The owner did not allow structural works on the house.)
Senhorio
B2This word specifically translates to "landlord" in the context of a lease. It describes a relationship; you cannot be a senhorio unless you have an inquilino (tenant). You use this word when talking about paying rent, requesting repairs, or discussing the tenancy contract.
O meu senhorio vem cá amanhã para arranjar a torneira.
(My landlord is coming over tomorrow to fix the tap.)
Tenho de pagar a renda ao senhorio antes do dia oito.
(I have to pay the rent to the landlord before the eighth.)
O senhorio não quis renovar o contrato de arrendamento.
(The landlord did not want to renew the rental contract.)
Tivemos uma discussão com o senhorio por causa do barulho.
(We had a discussion with the landlord because of the noise.)
É um senhorio muito simpático e nunca aumenta a renda.
(He is a very nice landlord and never raises the rent.)
Summary
In short, use proprietário when discussing legal ownership or the rights of an owner (property owner). Use senhorio when discussing the person to whom a tenant pays rent (landlord). Every senhorio is a proprietário, but a proprietário only becomes a senhorio once they rent their property to someone else.