Python datetime objects don't have time zone info by default, and without it, Python actually violates the ISO 8601 specification (if no time zone info is given, assumed to be local time). You can use the pytz package to get some default time zones, or directly subclass tzinfo yourself:

from datetime import datetime, tzinfo, timedelta
class simple_utc(tzinfo):
    def tzname(self,**kwargs):
        return "UTC"
    def utcoffset(self, dt):
        return timedelta(0)

Then you can manually add the time zone info to utcnow():

>>> datetime.utcnow().replace(tzinfo=simple_utc()).isoformat()
'2014-05-16T22:51:53.015001+00:00'

Note that this DOES conform to the ISO 8601 format, which allows for either Z or +00:00 as the suffix for UTC. Note that the latter actually conforms to the standard better, with how time zones are represented in general (UTC is a special case.)

ISO 8601 allows a compact representation with no separators except for the T, so I like to use this one-liner to get a quick timestamp string:

>>> datetime.datetime.now(datetime.UTC).strftime("%Y%m%dT%H%M%S.%fZ")
'20180905T140903.591680Z'

If you don't need the microseconds, just leave out the .%f part:

>>> datetime.datetime.now(datetime.UTC).strftime("%Y%m%dT%H%M%SZ")
'20180905T140903Z'

For local time:

>>> datetime.datetime.now(datetime.timezone(datetime.timedelta(hours=-5))).strftime("%Y-%m-%dT%H:%M:%S%:z")
'2018-09-05T14:09:03-05:00'

In general, I recommend you leave the punctuation in. RFC 3339 recommends that style because if everyone uses punctuation, there isn't a risk of things like multiple ISO 8601 strings being sorted in groups on their punctuation. So the one liner for a compliant string would be:

>>> datetime.datetime.now(datetime.UTC).strftime("%Y-%m-%dT%H:%M:%SZ")
'2018-09-05T14:09:03Z'

You need to make your datetime objects timezone aware. from the datetime docs:

There are two kinds of date and time objects: “naive” and “aware”. This distinction refers to whether the object has any notion of time zone, daylight saving time, or other kind of algorithmic or political time adjustment. Whether a naive datetime object represents Coordinated Universal Time (UTC), local time, or time in some other timezone is purely up to the program, just like it’s up to the program whether a particular number represents metres, miles, or mass. Naive datetime objects are easy to understand and to work with, at the cost of ignoring some aspects of reality.

When you have an aware datetime object, you can use isoformat() and get the output you need.

To make your datetime objects aware, you'll need to subclass tzinfo, like the second example in here, or simpler - use a package that does it for you, like pytz or python-dateutil

Using pytz, this would look like:

import datetime, pytz
datetime.datetime.now(timezone.utc).isoformat()

You can also control the output format, if you use strftime with the '%z' format directive like

datetime.datetime.now(timezone.utc).strftime('%Y-%m-%dT%H:%M:%S.%f%z')

Due to lack of reputation, I add this as new Answer.

Tomasz Swider Solution is a good start, however [:-3] will cut off seconds if the time provided does not have microseconds:

I will demonstrate this using utcfromtimestamp:

In [10]: datetime.utcfromtimestamp(0.1).isoformat()[:-3] + 'Z'
Out[10]: '1970-01-01T00:00:00.100Z'

In [10]: datetime.utcfromtimestamp(0).isoformat()[:-3] + 'Z'
Out[11]: '1970-01-01T00:00Z'

I think this is a cleaner solution to get an ISO Date with Milliseconds and 'Z' for Zulu time:

datetime.utcnow().isoformat(timespec='milliseconds')+ 'Z'

Again demonstrating using utcfromtimestamp:

In [25]: datetime.utcfromtimestamp(0.1).isoformat(timespec='milliseconds')+ 'Z'
Out[25]: '1970-01-01T00:00:00.100Z'

In [25]: datetime.utcfromtimestamp(0).isoformat(timespec='milliseconds')+ 'Z'
Out[25]: '1970-01-01T00:00:00.000Z'

Converting to ISO format: Python - Convert string representation of date to ISO 8601

Convert from ISO to UTC: How do I translate an ISO 8601 datetime string into a Python datetime object? to convert to datetime and How to convert local time string to UTC? to convert datetime to UTC.

(I didn't mark it as duplicate as there is no single post that answeres both question. Please notice you could easily google the answer)

Since Python 3.11, the standard library’s datetime.datetime.fromisoformat supports most valid ISO 8601 input (and some non-valid input, see docs). In earlier versions it only parses a specific subset, see the cautionary note at the end of the docs. If you are using Python 3.10 or earlier on strings that don't fall into that subset (like in the question), see other answers for functions from outside the standard library.

The current docs (so exceptions listed are still valid for Python 3.13):

classmethod datetime.fromisoformat(date_string):

Return a datetime corresponding to a date_string in any valid ISO 8601 format, with the following exceptions:

1. Time zone offsets may have fractional seconds.
2. The T separator may be replaced by any single unicode character.
3. Fractional hours and minutes are not supported.
4. Reduced precision dates are not currently supported (YYYY-MM, YYYY).
5. Extended date representations are not currently supported (±YYYYYY-MM-DD).
6. Ordinal dates are not currently supported (YYYY-OOO).

Examples:

>>> from datetime import datetime
>>> datetime.fromisoformat('2011-11-04')
datetime.datetime(2011, 11, 4, 0, 0)
>>> datetime.fromisoformat('20111104')
datetime.datetime(2011, 11, 4, 0, 0)
>>> datetime.fromisoformat('2011-11-04T00:05:23')
datetime.datetime(2011, 11, 4, 0, 5, 23)
>>> datetime.fromisoformat('2011-11-04T00:05:23Z')
datetime.datetime(2011, 11, 4, 0, 5, 23, tzinfo=datetime.timezone.utc)
>>> datetime.fromisoformat('20111104T000523')
datetime.datetime(2011, 11, 4, 0, 5, 23)
>>> datetime.fromisoformat('2011-W01-2T00:05:23.283')
datetime.datetime(2011, 1, 4, 0, 5, 23, 283000)
>>> datetime.fromisoformat('2011-11-04 00:05:23.283')
datetime.datetime(2011, 11, 4, 0, 5, 23, 283000)
>>> datetime.fromisoformat('2011-11-04 00:05:23.283+00:00')
datetime.datetime(2011, 11, 4, 0, 5, 23, 283000, tzinfo=datetime.timezone.utc)
>>> datetime.fromisoformat('2011-11-04T00:05:23+04:00')   
datetime.datetime(2011, 11, 4, 0, 5, 23, tzinfo=datetime.timezone(datetime.timedelta(seconds=14400)))

New in version 3.7.

Changed in version 3.11: Previously, this method only supported formats that could be emitted by date.isoformat() or datetime.isoformat().

If you only need dates, and not datetimes, you can use datetime.date.fromisoformat:

>>> from datetime import date
>>> date.fromisoformat("2024-01-31")
datetime.date(2024, 1, 31)