Can I say something to an ally when it's not my turn and we are both in combat?

Question

For instance I want to suggest a spell or a generic action to an ally because my PC has a better view of the battlefield. Can I do that or I must wait my turn?

Answer 1

Though I don't see disallowance specifically stated, the combat rules suggest that you cannot speak out-of-turn. From the SRD, italics mine:

Other Activity on Your Turn

...

You can communicate however you are able, through brief utterances and gestures, as you take your turn.

...

In my experience, however, it is quite common for DMs to ignore this rule. The community has brought up many reasons to ignore it:

• To allow the players to strategize for difficult or complicated combat encounters
• To keep combat engaging for players who are waiting for their turns to come up
• To encourage banter between opposing sides during combat

Even if your group decides to use this rule some of the above benefits can be had through a simple workaround. As pointed out by Foo Bar "[There is] a clean workaround in the 5E action economy: if you think of something out of turn and need to say it right then, it uses your reaction to do so. That fits the description on PHB p190 and turns kibitzing into a resource decision."

Answer 2

When my players want to convey messages to one another in the heat of combat, I'm one to say:

Okay, but their character doesn't know that.

There are a lot of things that are prevented from round to round, based on the strange turned-based combat we love. However, what doesn't make any sense is someone being unable to shout "Don't go that way they're lying in ambush!" when a character goes barreling in a dangerous direction.

So my players and I developed a solution by adding the following rule:

Any creature can use their reaction to convey a short message to any creature that can hear them.

This prevents long-winded combat strategies from being formed in the midst of combat while still believably allowing creatures to communicate with each other during lightning quick turns.

Answer 3

As Conduit stated in his/her answer, the rule clearly says No. However DM has the final say. And most of the time this rule is not followed.

So the answer to your question is "It depends on DM".

In our games, our DM strictly follows this rule. And restricts the speaking in turn, to a few short sentences. The reason behind that is preventing metagaming.